Cast: Ensure limited number of frames in flight
This cl makes sure that we don't have more than the max allowed frames in flight.
In addition, behavior when sender starts before receiver is
modified such that the sender will keep re-sending the first key frame instead of
generating (encoding) a new key frame.
A test and a DCHECK were added to ensure desired functionality.
BUG=347194
Review URL: https://codereview.chromium.org/
183093005
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254302 0039d316-1c4b-4281-b951-d872f2087c98