Multiple presubmit checks may call the same function multiple times, so it's
worth caching the results to speed up the presubmit check run.

Convert presubmit_support, git-cl and gcl to use it.

R=dpranke@chromium.org
BUG=


Review URL: https://chromiumcodereview.appspot.com/11280143

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@169726 0039d316-1c4b-4281-b951-d872f2087c98