GitLab

Implement the "seekto" and "scrubto" mojo commands
by calling the "seekto" action handler in Blink.

For reference, these are separate in the generic
media session mojo API because not all media
sessions support scrubbing.

BUG=977375

Change-Id: Ib62e069a03539e57de832e8108c877be7f582e93
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1697428Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683206}