Fix video-raf test flakiness

Despite hand coded test values, the difference between
base::TimeTicks::Now() and the Document's origin time can result in times
aligned with a 5 microseconds clock. This is a test failure, because we
don't know whether or not the time values were successfully clamped.

This CL uses the reference monotonic time instead of
base::TimeTicks::Now() when initializing the test values, which
guarantees deterministic results.

Bug: 1057319
Change-Id: I26ee485545a75ba25bc2e9187e78f032fce767e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2083737Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Auto-Submit: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746147}

third_party/blink/renderer/modules/video_raf/video_request_animation_frame_impl_test.cc

@@ -58,13 +58,12 @@ class MockFunction : public ScriptFunction {
 class MetadataHelper {
  public:
   static VideoFramePresentationMetadata* GetDefaultMedatada() {
-    InitializeFields();
+    DCHECK(initialized);
     return &metadata_;
   }
 
   static std::unique_ptr<VideoFramePresentationMetadata> CopyDefaultMedatada() {
-    InitializeFields();
-
+    DCHECK(initialized);
     auto copy = std::make_unique<VideoFramePresentationMetadata>();
 
     copy->presented_frames = metadata_.presented_frames;
@@ -78,8 +77,7 @@ class MetadataHelper {
     return copy;
   }
 
- private:
-  static void InitializeFields() {
+  static void InitializeFields(base::TimeTicks now) {
     if (initialized)
       return;
 
@@ -87,7 +85,6 @@ class MetadataHelper {
     // tell whether or not the implementation clamped their values. Therefore,
     // we manually set the values for a deterministic test, and make sure we
     // have sub-microsecond resolution for those values.
-    base::TimeTicks now = base::TimeTicks::Now();
 
     metadata_.presented_frames = 42;
     metadata_.presentation_time =
@@ -111,6 +108,7 @@ class MetadataHelper {
     initialized = true;
   }
 
+ private:
   static bool initialized;
   static VideoFramePresentationMetadata metadata_;
 };
@@ -344,6 +342,7 @@ TEST_F(VideoRequestAnimationFrameImplTest,
 
 TEST_F(VideoRequestAnimationFrameImplTest, VerifyParameters) {
   auto timing = GetDocument().Loader()->GetTiming();
+  MetadataHelper::InitializeFields(timing.ReferenceMonotonicTime());
 
   auto* callback =
       MakeGarbageCollected<VideoRafParameterVerifierCallback>(timing);