compute pressure: Adding supportedSources to API

To match existing observers API, supportedSources read-only property has been
added to the Compute Pressure interface.

https://wicg.github.io/compute-pressure/#the-supportedsources-attribute

It returns an array of supportedSource values supported by the user agent.
As the list of supported sources may vary per browser and will be evolving,
this property allows web developers to check which are available.

This patch doesn't affect the functionality of compute pressure.

Bug: 1292466
Change-Id: Ie1822dd90dcc4027624fc56d80424e6ea5484da1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3424278
Reviewed-by: Kentaro Hara <[email protected]>
Reviewed-by: Fr <[email protected]>
Reviewed-by: Raphael Kubo Da Costa <[email protected]>
Reviewed-by: Joshua Bell <[email protected]>
Commit-Queue: Arnaud Mandy <[email protected]>
Cr-Commit-Position: refs/heads/main@{#984321}

compute-pressure/compute_pressure_supported_sources.tentative.https.window.js

@@ -0,0 +1,19 @@
+'use strict';
+
+test(() => {
+  // Compute Pressure should support at least "cpu"
+  const sources = ComputePressureObserver.supportedSources;
+  assert_in_array('cpu', sources);
+}, 'ComputePressureObserver should support at least "cpu"');
+
+test(() => {
+  // Compute Pressure should be frozen array
+  const sources = ComputePressureObserver.supportedSources;
+  assert_equals(sources, ComputePressureObserver.supportedSources);
+}, 'ComputePressureObserver must return always the same array');
+
+test(() => {
+  // Compute Pressure should be frozen array
+  let sources = ComputePressureObserver.supportedSources;
+  assert_equals(Object.isFrozen(), true);
+}, 'ComputePressureObserver must return a frozen array');