Change MediaStreamManager to stop only affected devices when a USB device is unplugged and notify the render process.
Prior to this change we stopped all devices in a MediaStream if one device was unplugged. 


BUG=315585
TEST= Make sure you have a microphone, headset and a camera attached. On https://src.chromium.org/chrome/trunk/src/chrome/test/data/webrtc/manual/peerconnection.html click on request user media. 
Unplug the camera and make sure you can still hear yourself.

Review URL: https://codereview.chromium.org/75193003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237069 0039d316-1c4b-4281-b951-d872f2087c98
Delete ime_constants.h.
Move IBusMouseButton to Ibus_bridge.h.

BUG=N/A

Review URL: https://codereview.chromium.org/84943006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237070 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237069
SVN changes up to revision 237070