Fire canplaythrough event at the proper time for audio/video

In this CL, the browser fires the canplaythrough event based on an
approximation of the download speed of the media instead of firing the
event right away.

BUG=73609
TEST=NONE

Review URL: http://codereview.chromium.org/8399023

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@110733 0039d316-1c4b-4281-b951-d872f2087c98
20 files changed