BrowserCdmManager: gets RenderFrameHost instance on the correct thread.

This does not work if task_runner_ is not the browser UI thread.

R=xhwang@chromium.org
BUG=388930
TEST=manually verified EME playback

Review URL: https://codereview.chromium.org/730453004

Cr-Commit-Position: refs/heads/master@{#306907}