Do not call idle notification on context disposal.

The compositor calls regularly the idle notification. Hence, a direct invocation from the context disposal is not necessary.

BUG=397026

Review URL: https://codereview.chromium.org/606083003

git-svn-id: svn://svn.chromium.org/blink/trunk@182904 bbb929c8-8fbe-4397-9dbb-9b2b20218538