Roll chromium_revision 375f736..91f1781

Relevant changes:
* buildtools: 51ca1f2..c27f95b
* tools/gyp: b13d8f2..487c0b6
* tools/swarming_client: 41036ec..1f8ba35
Details: https://chromium.googlesource.com/chromium/src/+/375f736..91f1781/DEPS

Clang version was not updated in this roll.

BUG=
R=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/30049004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7680 4adac7df-926f-26a2-2b94-8c16560cd09d