Send UpdateCounters event after each GC

Otherwise counter graphs may not be updated for a long period after GC.

BUG=304689

Review URL: https://codereview.chromium.org/243303004

git-svn-id: svn://svn.chromium.org/blink/trunk@172025 bbb929c8-8fbe-4397-9dbb-9b2b20218538