Made SuspendableTimer smaller (removed redudant m_active field)

And fixed the FIXME in SuspendableTimer::resume()

Review URL: https://codereview.chromium.org/613163006

git-svn-id: svn://svn.chromium.org/blink/trunk@183986 bbb929c8-8fbe-4397-9dbb-9b2b20218538