Make FileReader's throttle controller a supplement.

Avoid using thread-local state to hold the per-ExecutionContext instance
of this controller object.

By doing so, we can also move the controller object to the heap with
Oilpan, enabling the tracing of the collections that the controller
keeps.

R=haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/395143002

git-svn-id: svn://svn.chromium.org/blink/trunk@178274 bbb929c8-8fbe-4397-9dbb-9b2b20218538