Restore logging of WMPI seek events in media log

There is already an existing MediaLog::CreateLoadEvent function, which
used to be called from WMPI, but seems to have been accidentally
removed in https://codereview.chromium.org/1641423002.
This CL will restore the SEEK event logging in media log and in
media-internals UI.

Review-Url: https://codereview.chromium.org/2756283002
Cr-Commit-Position: refs/heads/master@{#457940}