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CURRENT CODE:
The current flow of animations initiated by the ImageAnimationController
is as follows:

1) Each time a pending tree is updated, which is when animated images
from display lists would have been registered with the controller, we
compute the earliest time the next animation update will be needed and
schedule a delayed task to trigger an invalidation.

2) The delayed task triggers an invalidation, which update the frame on
the sync tree created as a result. And schedules the next invalidation
if needed.

THE BUG:
In the steps above, first for progressing the animation on the sync tree
and for scheduling the delayed task, we use the frame_time from the
BeginFrameArgs of the current frame as the "now" time. This resulted in
2 bugs:

1) Since the delayed task could be scheduled at any time within the impl
frame, using the frame_time from BeginFrameArgs as now was incorrect and
the delayed task would run at an unexpected time.

2) When the task did run at the correct time, resulting in an invalidation,
the pending tree would not be updated if the frame times of the animation
did not align with the BeginFrame frame times. For instance, if the frame
time is 0ms and the animation wants to run at 8ms. Even if the
invalidation runs at 8ms, the "now" time would be the BeginFrame
frame_time, which is 0ms and earlier than when the animation wants to run.
This would create wasted pending trees.

FIX:
This patch fixes the above by changing some fundamental aspects of how
image animations are progressed.

1) Instead of respecting the frame durations associated with the animated
image completely, snap them to frame boundaries. This ensures that the
animation always starts at the beginning of the frame in which the desired
time lies. This is also the optimal time to start the work for the frame
in which the animation wants to update.

2) Instead of always requesting an invalidation at the time for the next
animation update, request an impl frame instead. At the beginning of the
impl frame, upgrade it to an invalidation request if the frame_time for
that impl frame ensures that the animation will be updated.
This avoids a race between triggering an invalidation and receiving the
target BeginFrame in the renderer.

R=enne@chromium.org,sunnyps@chromium.org

Change-Id: Iffcaafc3dd72f7649ada1eceea2835054c44e911
Reviewed-on: https://chromium-review.googlesource.com/c/1464972
Commit-Queue: Khushal <khushalsagar@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Auto-Submit: Khushal <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634935}