Support FrameElement.reload() without an initial [src] attribute

The problem
---

If a `<turbo-frame>` element is rendered without a `[src]` attribute,
calls to `.reload()` will have no effect. If a `<turbo-frame>` is to be
its own browsing context, it should be able to apply its current
location (that is, it's owning document's current location) to its
browsing context.

For example, if a page has a `<turbo-frame>` element that contains text
that's typically updated by a Web Socket-delivered `<turbo-stream>`, it
might be useful to [gracefully degrade][] to periodic long-polling if
that Web Socket connection were to fail. That might involve something
like a `reload` Stimulus controller with a delay:

```html
<script type="module">
import { Application, Controller } from "@hotwired/stimulus"

const application = // boot up a Stimulus application

application.register("reload", class extends Controller {
  static values = { frequency: Number }

  disconnect() {
    this.#clearInterval()
  }

  frequencyValueChanged(frequencyInMilliseconds) {
    this.#clearInterval()

    this.intervalID = setInterval(() => this.element.reload(), frequencyInMilliseconds)
  }

  #clearInterval() {
    if (this.intervalID) clearInterval(this.intervalID)
  }
})
</script>

<turbo-frame id="dynamic-data" data-controller="reload" data-reload-frequency-value="30000">
  <h1>This data will refresh every 30 seconds</h1>
</turbo-frame>
```

The solution
---

When `FrameElement.reload()` is invoked, it delegates to its delegate
instance's `sourceURLReloaded()` method. In all cases,
`FrameElement.delegate` is an instance of `FrameController`.

This commit extends the `FrameController.sourceURLReloaded()`
implementation to set the element's `[src]` attribute to the element's
[baseURI][] value, which sets off the usual attribute change listeners
and `<turbo-frame>` navigation logic.

[baseURI]: https://developer.mozilla.org/en-US/docs/Web/API/Node/baseURI
[gracefully degrade]: https://developer.mozilla.org/en-US/docs/Glossary/Graceful_degradation

src/core/frames/frame_controller.js

@@ -94,7 +94,7 @@ export class FrameController {
       this.element.removeAttribute("complete")
     })
     this.element.src = null
-    this.element.src = src
+    this.element.src = src || this.element.baseURI
     return this.element.loaded
   }
 

src/tests/functional/frame_tests.js

@@ -515,15 +515,45 @@ test("test navigating a frame targeting _top from an outer link fires events", a
   assert.equal(otherEvents.length, 0, "no more events")
 })
 
-test("test invoking .reload() re-fetches the frame's content", async ({ page }) => {
+test("test invoking .reload() re-fetches the content of a <turbo-frame> element with a [src] attribute", async ({ page }) => {
   await page.click("#link-frame")
   await nextEventOnTarget(page, "frame", "turbo:frame-load")
   await page.evaluate(() => document.getElementById("frame").reload())
 
   const dispatchedEvents = await readEventLogs(page)
 
   assert.deepEqual(
-    dispatchedEvents.map(([name, _, id]) => [id, name]),
+    dispatchedEvents
+      .map(([name, _, id]) => [id, name])
+      .filter(([id]) => id === "frame"),
+    [
+      ["frame", "turbo:before-fetch-request"],
+      ["frame", "turbo:before-fetch-response"],
+      ["frame", "turbo:before-frame-render"],
+      ["frame", "turbo:frame-render"],
+      ["frame", "turbo:frame-load"]
+    ]
+  )
+})
+
+test("test invoking .reload() re-fetches the content of a <turbo-frame> element without a [src] attribute", async ({ page }) => {
+  const frame = await page.locator("turbo-frame#frame")
+  const heading = await frame.locator("h2")
+
+  assert.match(await heading.textContent(), /Frames: #frame/)
+
+  await heading.evaluate((element) => element.textContent = "Not yet refreshed")
+
+  assert.match(await heading.textContent(), /Not yet refreshed/)
+
+  await frame.evaluate((element) => element.reload())
+
+  const dispatchedEvents = await readEventLogs(page)
+
+  assert.deepEqual(
+    dispatchedEvents
+      .map(([name, _, id]) => [id, name])
+      .filter(([id]) => id === "frame"),
     [
       ["frame", "turbo:before-fetch-request"],
       ["frame", "turbo:before-fetch-response"],