Speculative fix for LayerTreeHostCustomThrougputTrackerTest flakes

EndTest() only really exit tests when there are no pending main
frames. This might be cause of flaky timeout failure. This CL
changes to redraw only the needed frames so that EndTest() would
not be blocked.

Bug: 1127563
Change-Id: I28943ddf1bd247b1ec2cdebc98fb28d77b2f2543
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2426971
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811377}

cc/trees/layer_tree_host_unittest.cc

@@ -8826,6 +8826,7 @@ class LayerTreeHostCustomThrougputTrackerTest : public LayerTreeHostTest {
         break;
     }
 
+    if (!TestEnded())
       PostSetNeedsCommitWithForcedRedrawToMainThread();
   }