Wire up CPU adaptation in WebRtcVideoEngine2.

Includes clean-up work to be able to use the webrtc::Call::Config that's
set up. This introduced a CallFactory to spawn a FakeCall with the
config used and allowed removal of FakeWebRtcVideoChannel2.

BUG=1788
R=mflodman@webrtc.org, pthatcher@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/24779004

git-svn-id: http://webrtc.googlecode.com/svn@7370 4adac7df-926f-26a2-2b94-8c16560cd09d
Only try to do CPU detection on Android ARM.

Previously, the code made the assumption that ARM implied Android. It
uses Android-specific machinery to do run-time NEON detection. Retain
this functionality, but on non-Android, don't do run-time detection;
instead require arm_neon==1 in order to build the NEON bits.

BUG=chromium:415393
TEST=Production code requiring openmax_dl runs correctly on an ARM
Chromebook.

R=rtoy@google.com

Review URL: https://webrtc-codereview.appspot.com/29639004

git-svn-id: http://webrtc.googlecode.com/svn@7371 4adac7df-926f-26a2-2b94-8c16560cd09d