Call rAFs before checking the pinch zoom result

This test failing because it need some rAFs for commit.

Bug: 875097
Change-Id: I10aacf4a55ea8fe277fef1fc68b4d6c9ee287190
Reviewed-on: https://chromium-review.googlesource.com/1213545Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Jianpeng Chao <chaopeng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589562}

third_party/WebKit/LayoutTests/virtual/threaded/synthetic_gestures/synthetic-pinch-zoom-gesture.js

@@ -18,6 +18,7 @@ function runTest(t, testCase) {
     waitForCompositorCommit().then(t.step_func(() => {
       chrome.gpuBenchmarking.pinchBy(
         testCase.scale, centerX, centerY, t.step_func(() => {
+          waitForCompositorCommit().then(t.step_func(() => {
             const expectedScale = testCase.startingScale * testCase.scale;
             assert_approx_equals(
               window.visualViewport.scale,
@@ -35,6 +36,7 @@ function runTest(t, testCase) {
               kOffsetEpsilon,
               testCase.msg + " has correct visual viewport offsetTop.");
             resolve();
+          }))
         }), testCase.speed, testCase.gestureSource);
     }));
   }));