Download and use GN similar to how WebRTC uses it
for GYP/ninja generation
Similar changes as done in
https://webrtc-codereview.appspot.com/6939004

Add support for VS2013 compilation using the
Chromium standalone toolchain. Similar changes:
https://webrtc-codereview.appspot.com/9769004

Fix the valgrind wrapper script similar to
https://webrtc-codereview.appspot.com/5589006
https://webrtc-codereview.appspot.com/6939004

Remove no longer needed cygwin mount hook.

TEST=All trybots except iOS are passing.
Those will have to be fixed in a future CL.
BUG=310
TESTED=gclient sync on osx/ios
R=tpsiaki@google.com

Review URL: https://webrtc-codereview.appspot.com/10849004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@991 16f28f9a-4ce2-e073-06de-1de4eb20be90