The current DEFAULT_BLACKLIST in presubmit_support blacklists by
default third_party/.* in every repo. This requires an adjustement
after the blink merge, as it causing all the blink presubmits to
become noops.

BUG=539768

Review URL: https://codereview.chromium.org/1382993004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@297030 0039d316-1c4b-4281-b951-d872f2087c98