Show controls and update time indicators during seeking

This commit modifies the MediaControls to show the controls and update
the current time indicators (current time text + timeline thumb)
immediately during seek requests so the element behavior during rapid
seek requests (e.g. user is holding down the FF button on their BT
keyboard) look similar to when a user is scrubbing the timeline with
their mouse.

Bug: 748749

TEST=MediaControlsImplTest, manual testing
     out/Debug/bin/run_webkit_unit_tests -f *MediaControlsImplTest*

Change-Id: I6588aded2a93e081a6ca638ee1da534ea00abf4d
Reviewed-on: https://chromium-review.googlesource.com/c/1274945
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636493}
8 files changed