Support sending MIDI data with throttling

The Web MIDI API allows sending of MIDI data, but we need to take
care to not send too much data from the renderer to the browser.
We implement a throttling mechanism where the browser process
acknowledges the number of sent bytes back to the renderer,
and the renderer only sends new data if it hasn't already sent
too much unacknowledged data.

BUG=163795
TEST=none

Review URL: https://chromiumcodereview.appspot.com/19612004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@214570 0039d316-1c4b-4281-b951-d872f2087c98
9 files changed