GitLab

Pipeline generates OnPipelineSeeked and OnPipelineBufferingStateChanged
events in response to seek operations. There is an event generated already
for OnPipelineBufferingStateChanged but missing one for OnPipelineSeeked.

Follow up fix for https://codereview.chromium.org/740663002/
Required for landing https://codereview.chromium.org/456343002/

Review URL: https://codereview.chromium.org/791233003

Cr-Commit-Position: refs/heads/master@{#308263}