Add clang-format to presubmit checks

The check doesn't modify the patch, instead, you'll have to use "git cl
format" manually.

BUG=none
R=danno@google.com, danno@chromium.org, jkummerow@chromium.org
LOG=y

Review URL: https://codereview.chromium.org/362263002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22165 ce2b1a6d-e550-0410-aec6-3dcde31c8c00