[TaskScheduler] Handle early first paint in NoBackgroundTasksTest.

NoBackgroundTasksTest.FirstNonEmptyPaintWithoutBackgroundTasks is a
test that verifies that the first paint in a tab can happen when
TaskPriority::BEST_EFFORT tasks don't run.

Before this CL, the test's body ran a RunLoop until
WebContentsObserver::DidFirstVisuallyNonEmptyPaint() was invoked.
A timeout occured if the first paint occured before the test's body
was entered. Having the first paint before the test's body is
possible, because InProcessBrowserTest::PreRunTestOnMainThread()
runs tasks before jumping to the test's body (see
https://cs.chromium.org/chromium/src/chrome/test/base/in_process_browser_test.cc?l=486&rcl=8edfdb37ebe40b1ba4dae27a23d8edaeac09a0ee).

This CL fixes the issue by skipping running the RunLoop if
WebContents::CompletedFirstVisuallyNonEmptyPaint() is already
true at the beginning of the test's body.

This is one of multiple fixes that will allow us to enable the
test on ChromeOS.

Bug: 833989
Change-Id: I0ce024bda351059c4e0ba8648e1f0f0459eb19ac
Reviewed-on: https://chromium-review.googlesource.com/1187131Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585884}

chrome/browser/no_background_tasks_browsertest.cc

@@ -25,7 +25,11 @@ class RunLoopUntilNonEmptyPaint : public content::WebContentsObserver {
 
   // Runs a RunLoop on the main thread until the first non-empty frame is
   // painted for the WebContents provided to the constructor.
-  void RunUntilIdle() { run_loop_.Run(); }
+  void RunUntilNonEmptyPaint() {
+    if (web_contents()->CompletedFirstVisuallyNonEmptyPaint())
+      return;
+    run_loop_.Run();
+  }
 
  private:
   // content::WebContentsObserver:
@@ -70,5 +74,5 @@ IN_PROC_BROWSER_TEST_F(NoBackgroundTasksTest,
                        MAYBE_FirstNonEmptyPaintWithoutBackgroundTasks) {
   RunLoopUntilNonEmptyPaint run_loop_until_non_empty_paint(
       browser()->tab_strip_model()->GetActiveWebContents());
-  run_loop_until_non_empty_paint.RunUntilIdle();
+  run_loop_until_non_empty_paint.RunUntilNonEmptyPaint();
 }

content/browser/web_contents/web_contents_impl.cc

@@ -6143,12 +6143,12 @@ bool WebContentsImpl::GetAllowOtherViews() {
   return view_->GetAllowOtherViews();
 }
 
+#endif
+
 bool WebContentsImpl::CompletedFirstVisuallyNonEmptyPaint() const {
   return did_first_visually_non_empty_paint_;
 }
 
-#endif
-
 void WebContentsImpl::OnDidDownloadImage(
     ImageDownloadCallback callback,
     int id,

content/browser/web_contents/web_contents_impl.h

@@ -455,6 +455,7 @@ class CONTENT_EXPORT WebContentsImpl : public WebContents,
   void SetShowingContextMenu(bool showing) override;
   void PausePageScheduledTasks(bool paused) override;
   BrowserPluginGuest* GetBrowserPluginGuest() const override;
+  bool CompletedFirstVisuallyNonEmptyPaint() const override;
 
 #if defined(OS_ANDROID)
   base::android::ScopedJavaLocalRef<jobject> GetJavaWebContents() override;
@@ -465,7 +466,6 @@ class CONTENT_EXPORT WebContentsImpl : public WebContents,
 #elif defined(OS_MACOSX)
   void SetAllowOtherViews(bool allow) override;
   bool GetAllowOtherViews() override;
-  bool CompletedFirstVisuallyNonEmptyPaint() const override;
 #endif
 
   bool HasRecentInteractiveInputEvent() const override;

content/public/browser/web_contents.h

@@ -916,10 +916,11 @@ class WebContents : public PageNavigator,
 
   // Returns true if other views are allowed, false otherwise.
   virtual bool GetAllowOtherViews() = 0;
+#endif  // OS_ANDROID
 
-  // Returns true if the WebContents has completed its first meaningful paint.
+  // Returns true if the WebContents has completed its first meaningful paint
+  // since the last navigation.
   virtual bool CompletedFirstVisuallyNonEmptyPaint() const = 0;
-#endif  // OS_ANDROID
 
   // TODO(https://crbug.com/826293): This is a simple mitigation to validate
   // that an action that requires a user gesture actually has one in the