This makes the style use CamelCase instead of ALL_CAPS. It also cleans
up some of the naming. It also changes some uses of the messages in
some of the objects to hopefully be more consistent. See the comments
added to the enum SubjectMessage in Observer.h for more details.

Bug: angleproject:3427
Change-Id: I6dff4f6d335ecf1a27e48df65743b1490bd3025a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1600411
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>