Simplify GCForContextDispose

Instead of using a timer and sending an idle notification in any case
after 800ms, only send the idle notification if we detect some idle time
shortly after the context disposal.

BUG=none
R=hpayer@chromium.org,erik.corry@gmail.com,rmcilroy@chromium.org

Review URL: https://codereview.chromium.org/641023002

git-svn-id: svn://svn.chromium.org/blink/trunk@183461 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add haraken@ to Source/web/OWNERS

Review URL: https://codereview.chromium.org/631423002

git-svn-id: svn://svn.chromium.org/blink/trunk@183462 bbb929c8-8fbe-4397-9dbb-9b2b20218538