Pass the proper RenderFrameHost to observers.

BUG=347339

Review URL: https://codereview.chromium.org/181773008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254009 0039d316-1c4b-4281-b951-d872f2087c98
Enable round-tripping and updating of WebSourceBufferImpl timestamp offset

This is the first part of a three-sided MediaSource
Chromium/Blink+roll Blink/Chromium set of changes to enable
JavaScript web apps to continue to round-trip
SourceBuffer.timestampOffset set/get if SourceBuffer append
operations do not update the timestamp offset, and to
retrieve any updated timestamp offset as can occur during
upcoming "sequence" append mode compliant coded frame
processing.

R=acolwell@chromium.org
BUG=249422
TEST=All media_unittests and http/tests/media/media-source layout tests pass locally on Linux

Review URL: https://codereview.chromium.org/178153004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254010 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254010