This simplifies the state management code inside this manager. Also
it should improve the speed of lookups due to using hashing instead
of a tree lookup.

BUG=angleproject:1156
BUG=angleproject:2044

Change-Id: I19ea8dbac6f2dfd7d30dd403d77b66ba0aa85d73
Reviewed-on: https://chromium-review.googlesource.com/527693
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>