This patch makes my_activity.py use the new REST API to access the
list of gerrit changes. The client doesn't authenticate with the
server, so only the public changes are shown.

BUG=chromium:281695
TEST=Ran my_activity.py, shows changes&reviews and links work.

Review URL: https://chromiumcodereview.appspot.com/24047003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@222767 0039d316-1c4b-4281-b951-d872f2087c98