[LayoutTests] Make display compositor pixel dumps also support threaded compositing.

If enable-threaded-compositing is specified (via a threaded virtual test,
or on the command line), along with the display compositor dumps, then
we don't need to synchronously composite frames. Instead we should rely
on surface synchronization to wait for frames to be submitted via
a scheduler.

R=piman@chromium.org, fsamuel@chromium.org

Bug: 667551
Change-Id: I05859b1ebe18cc59379514d604268940714250b0
Reviewed-on: https://chromium-review.googlesource.com/1026563Reviewed-by: Fady Samuel <fsamuel@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553885}

content/shell/browser/layout_test/blink_test_controller.cc

@@ -597,11 +597,19 @@ void BlinkTestController::OnInitiateCaptureDump(bool capture_navigation_history,
         switches::kEnableDisplayCompositorPixelDump));
     waiting_for_pixel_results_ = true;
 
-    // Trigger compositing on all frames.
-    CompositeAllFrames();
+    auto* rwhv = main_window_->web_contents()->GetRenderWidgetHostView();
+    // If we're running in threaded mode, then the frames will be produced via a
+    // scheduler elsewhere, all we need to do is to ensure that the surface is
+    // synchronized before we copy from it. In single threaded mode, we have to
+    // force each renderer to produce a frame.
+    if (base::CommandLine::ForCurrentProcess()->HasSwitch(
+            switches::kEnableThreadedCompositing)) {
+      rwhv->EnsureSurfaceSynchronizedForLayoutTest();
+    } else {
+      CompositeAllFrames();
+    }
 
     // Enqueue a copy output request.
-    auto* rwhv = main_window_->web_contents()->GetRenderWidgetHostView();
     rwhv->CopyFromSurface(
         gfx::Rect(), gfx::Size(),
         base::BindOnce(&BlinkTestController::OnPixelDumpCaptured,