Make FrameTestHelpers::runPendingTasks wait for Timers.

ThreadTimers::sharedTimerFiredInternal() only runs Timers for 50ms before
yielding to the MessageLoop.  On heavily-loaded bots, this meant that
runPendingTasks wasn't enough to wait for components that use 0-delay Timers to
yield to the MessageLoop.

This deflakes WebFrameCSSCallbackTest: CSSSelectorWatch uses a 0-delay Timer,
and the test wants to wait for it to complete.

BUG=306952

Review URL: https://codereview.chromium.org/59613008

git-svn-id: svn://svn.chromium.org/blink/trunk@161421 bbb929c8-8fbe-4397-9dbb-9b2b20218538