Fix flaky LayerTreeHostTestStartPageScaleAnimation test.

cc/trees/layer_tree_host_unittest.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/layer_tree_host.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 2454 matching lines @@
   void DidActivateTreeOnThread(LayerTreeHostImpl* impl) override {
     // We get one commit before the first draw, and the animation doesn't happen
     // until the second draw.
     switch (impl->active_tree()->source_frame_number()) {
       case 0:
         EXPECT_EQ(1.f, impl->active_tree()->current_page_scale_factor());
         // We'll start an animation when we get back to the main thread.
         break;
       case 1:
         EXPECT_EQ(1.f, impl->active_tree()->current_page_scale_factor());
+        // Once the animation starts, an ImplFrame will be requested. However,
+        // main frames may be happening in the mean-time due to high-latency
+        // mode. If one happens before the next impl frame, then the source
+        // frame number may increment twice instead of just once.
         break;
       case 2:
+      case 3:
         EXPECT_EQ(1.25f, impl->active_tree()->current_page_scale_factor());
         EndTest();
         break;
-      case 3:
-        break;
-      default:
-        NOTREACHED();
     }
   }
 
   void DidCommitAndDrawFrame() override {
     switch (layer_tree_host()->SourceFrameNumber()) {
       case 1:
         layer_tree_host()->StartPageScaleAnimation(gfx::Vector2d(), false,
                                                    1.25f, base::TimeDelta());
         break;
     }
@@ skipping 5151 matching lines @@
   void AfterTest() override {}
 
  private:
   bool received_ack_ = false;
 };
 
 SINGLE_AND_MULTI_THREAD_TEST_F(LayerTreeHostTestDiscardAckAfterRelease);
 
 }  // namespace
 }  // namespace cc