PDF Viewer: Animate highlight when switching tabs

Bug: 1048812
Change-Id: I4253e49562b4d0afb07dcccebc6f6e4ce0cc5e1b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2393157


Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805542}