Let WebMediaPlayerClient seek the MediaPlayerClient

This Blink change is a prerequisite to fixing seek
coordination in later CLs. This change lets
WebMediaPlayers trigger seeks.

BUG=267111
TEST=all existing media and http/tests/media layout tests pass on desktop linux
R=acolwell@chromium.org, jamesr@chromium.org, adamk@chromium.org, qinmin@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23351006

git-svn-id: svn://svn.chromium.org/blink/trunk@156595 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Revert 156582 "Merge FrameLoader::stopLoading into stopAllLoaders"

> Merge FrameLoader::stopLoading into stopAllLoaders
> 
> FrameLoader::stopLoading is called twice:
> * From FrameLoader::closeURL after the unload event, when the relevant document is about to disappear and is going to get cleaned up anyway.
> * From DocumentLoader::stopLoading in the case where the document is committed. The only time this occurs is within FrameLoader::stopAllLoaders.
> 
> BUG=none
> 
> Review URL: https://chromiumcodereview.appspot.com/23361010

TBR=japhet@chromium.org

Review URL: https://codereview.chromium.org/23020029

git-svn-id: svn://svn.chromium.org/blink/trunk@156596 bbb929c8-8fbe-4397-9dbb-9b2b20218538