Instead of storing a binary flag indicating if we've created
immutable storage, store a level count. This indicates if or if
not we've created immutable storage, as well as avoiding the
call to the native storage to return the level count.

BUG=angle:781

Change-Id: I08a6bd086d00ddf3255262a7711628be5212b4b7
Reviewed-on: https://chromium-review.googlesource.com/222921Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>