Pass seek event through pipeline during loading state

When try to land https://codereviougew.chromium.org/456343002/ the test
controls-restrained-media-controller.html is failing sometimes as the
frame is not ready before the canplay event. Reason being canplay event
is fired before VideoFrameCompositor::UpdateCurrentFrame if the seek event
is not passed to pipeline during loading state. Also removed
OnPipelineSeeked which is redundant now.

Required for landing https://codereview.chromium.org/456343002/

Review URL: https://codereview.chromium.org/787373008

Cr-Commit-Position: refs/heads/master@{#310483}
1 file changed