Instead of having a direct/static observer distinction, add two
messages for 'Contents Changed' and 'Storage Changed'. This makes
Buffer11 itself the subject with two different message handling
cases in the onSubjectStateChange methods.

Bug: angleproject:2389

Change-Id: I645cd4b7cc7ce51cb7f48a01c7fc72939cbe89fe
Reviewed-on: https://chromium-review.googlesource.com/957940
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>