Unittest for Component updater throttles

Part of the on-demand feature see
https://codereview.chromium.org/25713007/ for more context.

BUG=none
TEST=none
R=sorin@chromium.org, waffles@chromium.org

Review URL: https://codereview.chromium.org/55303002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236892 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236892