Details: https://chromium.googlesource.com/chromium/src/+/271c6cc..2c3ffb2/DEPS

Updated sync_chromium.py to be identical with WebRTC's current version.
Removed the xcode61 suffixed Mac trybots as well.

The following were moved into src/buildtools:
* src/third_party/libc++/trunk
* src/third_party/libc++abi/trunk

TBR=fbarchard@google.com
TESTED=All trybots passing this.

Review URL: https://webrtc-codereview.appspot.com/40119004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1301 16f28f9a-4ce2-e073-06de-1de4eb20be90