FrameMetrics: Add more comments to unittests

Bug: 894149

TBR=sadrul@chromium.org

Change-Id: Ibeece84bc1490050a9c61c8deb976c370506e45e
Reviewed-on: https://chromium-review.googlesource.com/c/1335279Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Yi Xu <yiyix@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608016}

ui/latency/skipped_frame_tracker_unittest.cc

@@ -394,9 +394,14 @@ TEST_F(SkippedFrameTrackerTest, NoSkips_ActiveIdleActive_FramePulledIsPush) {
   EXPECT_FALSE(tracker_.IsActive());
 }
 
+// Simulate that SetNeedsRedraw is called, then the client realized that it
+// doesn't need a new BeginFrame.
 TEST_F(SkippedFrameTrackerTest, NoFrameProduced) {
   EXPECT_TRUE(WillProduceFrame());
   EXPECT_TRUE(WillNotProduceFrame());
+
+  // Since no BeginFrame is needed, number of frames produced and the number
+  // of skipped frames should all be 0.
   EXPECT_EQ(0, client_.amount_produced_);
   EXPECT_EQ(0, client_.amount_skipped_);
 }