Add more cc animation tracing.

This cl adds tracing to see why ScrollAnimationUpdateTarget returns false.

Bug: 797708
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I40b0e2b71d25ddaa4a0d685cfb223bd83dd8f1b2
Reviewed-on: https://chromium-review.googlesource.com/1073754
Commit-Queue: Sahel Sharifymoghaddam <sahel@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562057}

cc/animation/scroll_offset_animations_impl.cc

@@ -4,6 +4,7 @@
 
 #include "cc/animation/scroll_offset_animations_impl.h"
 
+#include "base/trace_event/trace_event.h"
 #include "cc/animation/animation_host.h"
 #include "cc/animation/animation_id_provider.h"
 #include "cc/animation/animation_timeline.h"
@@ -66,8 +67,11 @@ bool ScrollOffsetAnimationsImpl::ScrollAnimationUpdateTarget(
     base::TimeTicks frame_monotonic_time,
     base::TimeDelta delayed_by) {
   DCHECK(scroll_offset_animation_);
-  if (!scroll_offset_animation_->has_element_animations())
+  if (!scroll_offset_animation_->has_element_animations()) {
+    TRACE_EVENT_INSTANT0("cc", "No element animation exists",
+                         TRACE_EVENT_SCOPE_THREAD);
     return false;
+  }
 
   DCHECK_EQ(element_id, scroll_offset_animation_->element_id());
 
@@ -75,6 +79,8 @@ bool ScrollOffsetAnimationsImpl::ScrollAnimationUpdateTarget(
       scroll_offset_animation_->GetKeyframeModel(TargetProperty::SCROLL_OFFSET);
   if (!keyframe_model) {
     scroll_offset_animation_->DetachElement();
+    TRACE_EVENT_INSTANT0("cc", "No keyframe model exists",
+                         TRACE_EVENT_SCOPE_THREAD);
     return false;
   }
   if (scroll_delta.IsZero())