Specify V8 branch through a DEPS variable.

This allows to specify other branches like "branches/bleeding_edge" without overwriting the whole deps entry.

BUG=
R=agable@chromium.org, jkummerow@chromium.org

Review URL: https://codereview.chromium.org/185373004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254916 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254916