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Improve comments for ExtensionWebRequestApiTest.WebRequestApiDoesNotCrashOnErrorAfterProfileDestroyed

Original comments were a little incorrect.

Change-Id: Icb52d1e83ae176be5b019ec9b5874e9193f72c8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1887430Reviewed-by: Dominic Battré <battre@chromium.org>
Commit-Queue: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711270}

chrome/browser/extensions/api/web_request/web_request_apitest.cc

@@ -1911,9 +1911,12 @@ IN_PROC_BROWSER_TEST_F(ExtensionWebRequestApiTest,
 
   // We are about to destroy a profile. In production that will only happen
   // as part of the destruction of BrowserProcess's ProfileManager. This
-  // happens in ShutdownPostThreadsStop(). This means that to have this test
-  // represent production we have to make sure that no tasks are pending before
-  // we destroy the profile.
+  // happens in PostMainMessageLoopRun(). This means that to have this test
+  // represent production we have to make sure that no tasks are pending on the
+  // main thread before we destroy the profile. We also would need to prohibit
+  // the posting of new tasks on the main thread as in production the main
+  // thread's message loop will not be accepting them. We fallback on flushing
+  // the ThreadPool here to avoid the posts coming from it.
   content::RunAllTasksUntilIdle();
 
   ProfileDestroyer::DestroyProfileWhenAppropriate(temp_profile);