Add kGoogEchoCancellation to MediaConstraintsInterface.

Add kGoogEchoCancellation to MediaConstraintsInterface.
This constraint will be equal to kEchoCancellation until we've updated Chromium to use kGoogEchoCancellation where that constraint is needed.  Once that's done, I'll change kEchoCancellation to be 'echoCancellation'.

BUG=webrtc:4747
R=andrew@webrtc.org

Committed: https://chromium.googlesource.com/external/webrtc/+/70c7fe14ac322a3abac2726d29ae3264b956e3da

[Andrew MacDonald, 2015-06-15 06:56:53]

lgtm

[Andrew MacDonald, 2015-06-15 06:58:54]

https://codereview.webrtc.org/1179233003/diff/1/talk/app/webrtc/mediaconstrai...
File talk/app/webrtc/mediaconstraintsinterface.cc (right):

https://codereview.webrtc.org/1179233003/diff/1/talk/app/webrtc/mediaconstrai...
talk/app/webrtc/mediaconstraintsinterface.cc:51: const char
MediaConstraintsInterface::kGoogEchoCancellation[] =
Not for this CL of course, but should we change the other "goog" flags to be
similarly Goog prefixed? Not sure it's worth the pain ;)

[tommi, 2015-06-15 07:12:51]

https://codereview.webrtc.org/1179233003/diff/1/talk/app/webrtc/mediaconstrai...
File talk/app/webrtc/mediaconstraintsinterface.cc (right):

https://codereview.webrtc.org/1179233003/diff/1/talk/app/webrtc/mediaconstrai...
talk/app/webrtc/mediaconstraintsinterface.cc:51: const char
MediaConstraintsInterface::kGoogEchoCancellation[] =
On 2015/06/15 at 06:58:54, andrew wrote:
> Not for this CL of course, but should we change the other "goog" flags to be
similarly Goog prefixed? Not sure it's worth the pain ;)

Don't know if it's worth spending the time on it right now but yes it would be
good to be consistent.  I'll keep it in mind with future CLs I see in this area.
I saw a todo for removing googEchoCancellation2.  Maybe there's some cleanup
needed.

[tommi, 2015-06-15 07:14:22]

Committed patchset #1 (id:1) manually as
70c7fe14ac322a3abac2726d29ae3264b956e3da (presubmit successful).