Don't update the local source constraints on WebAudio creation. 

WebAudio should not be able to specify media constraints; this removes 
that possibility, instead passing NULL when creating the local audio 
source. 

Also:
- Apply the default and platform effects constraint changes to the
optional rather than mandatory group.
- Abstract the harmonization of constraints and platform effects 
to a function. This will ease future refactoring. 

TESTED=apprtc behaves as before wrt constraints and effects.

Review URL: https://codereview.chromium.org/132353005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250084 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250084
drive: Cancel fetch sync task when unpinning a file

BUG=248856
TEST=Open Google Drive directory in Files.app, right click a file and check "Available offline", wait until the sync icon appears on the system tray, uncheck "Available offline", ensure that the sync task is canceled and the sync icon disappears.

Review URL: https://codereview.chromium.org/158683002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250085 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250085