Bug 1363830 - use previous frame time when advancing animations, r=kats

MozReview-Commit-ID: CqKOguZXnF9

--HG--
extra : rebase_source : b1ba85d20978fe5e7853a07065df7e3626c4a482

gfx/layers/wr/WebRenderBridgeParent.cpp

@@ -1053,7 +1053,16 @@ WebRenderBridgeParent::AdvanceAnimations()
   if (CompositorBridgeParent* cbp = GetRootCompositorBridgeParent()) {
     animTime = cbp->GetTestingTimeStamp().valueOr(animTime);
   }
-  AnimationHelper::SampleAnimations(mAnimStorage, animTime);
+
+  AnimationHelper::SampleAnimations(mAnimStorage,
+                                    !mPreviousFrameTimeStamp.IsNull() ?
+                                    mPreviousFrameTimeStamp : animTime);
+
+  // Reset the previous time stamp if we don't already have any running
+  // animations to avoid using the time which is far behind for newly
+  // started animations.
+  mPreviousFrameTimeStamp =
+    mAnimStorage->AnimatedValueCount() ? animTime : TimeStamp();
 }
 
 void

gfx/layers/wr/WebRenderBridgeParent.h

@@ -281,6 +281,7 @@ private:
   nsDataHashtable<nsUint64HashKey, RefPtr<WebRenderImageHost>> mExternalImageIds;
   nsTArray<ImageCompositeNotificationInfo> mImageCompositeNotifications;
 
+  TimeStamp mPreviousFrameTimeStamp;
   // These fields keep track of the latest layer observer epoch values in the child and the
   // parent. mChildLayerObserverEpoch is the latest epoch value received from the child.
   // mParentLayerObserverEpoch is the latest epoch value that we have told TabParent about