Add checks in ScheduleAnimation to help understand crash

The WebFrameWidget always has a client while the WebLocalFrameImpl
points to the WebFrameWidget. So let's try fish out what assumptions
are wrong.

R=dcheng@chromium.org

Bug: 939262
Change-Id: I6de73c4dd29ecfdccb9d6529b5e3763c5adadd30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1510012
Commit-Queue: danakj <danakj@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639112}
3 files changed