fix(webkit): do not swallow init errors (#2242)

This is a speculative fix to the following issue from the bots:

NON-TEST ERROR #0: UNHANDLED ERROR
  TypeError: Cannot read property 'url' of undefined
      at WKPage._onTargetCreated (/Users/runner/runners/2.169.1/work/playwright/playwright/src/webkit/wkPage.ts:274:12)
      at process._tickCallback (internal/process/next_tick.js:68:7)

I assume that _initializeSession did swallow an error, so we erroneously
consider Page to be fully initialized (and having main frame).

src/webkit/wkPage.ts

@@ -119,9 +119,11 @@ export class WKPage implements PageDelegate {
 
   // This method is called for provisional targets as well. The session passed as the parameter
   // may be different from the current session and may be destroyed without becoming current.
-  async _initializeSession(session: WKSession, resourceTreeHandler: (r: Protocol.Page.getResourceTreeReturnValue) => void) {
+  async _initializeSession(session: WKSession, provisional: boolean, resourceTreeHandler: (r: Protocol.Page.getResourceTreeReturnValue) => void) {
     await this._initializeSessionMayThrow(session, resourceTreeHandler).catch(e => {
-      if (session.isDisposed())
+      // Provisional session can be disposed at any time, for example due to new navigation initiating
+      // a new provisional page.
+      if (provisional && session.isDisposed())
         return;
       // Swallow initialization errors due to newer target swap in,
       // since we will reinitialize again.
@@ -263,7 +265,7 @@ export class WKPage implements PageDelegate {
         this._setSession(session);
         await Promise.all([
           this._initializePageProxySession(),
-          this._initializeSession(session, ({frameTree}) => this._handleFrameTree(frameTree)),
+          this._initializeSession(session, false, ({frameTree}) => this._handleFrameTree(frameTree)),
         ]);
         pageOrError = this._page;
       } catch (e) {

src/webkit/wkProvisionalPage.ts

@@ -48,7 +48,7 @@ export class WKProvisionalPage {
       helper.addEventListener(session, 'Network.loadingFailed', overrideFrameId(e => wkPage._onLoadingFailed(e))),
     ];
 
-    this.initializationPromise = this._wkPage._initializeSession(session, ({frameTree}) => this._handleFrameTree(frameTree));
+    this.initializationPromise = this._wkPage._initializeSession(session, true, ({frameTree}) => this._handleFrameTree(frameTree));
   }
 
   dispose() {