Removes several TODOs. Only egl::Error remains.

Also slightly decreases binary size.

Bug: angleproject:2491
Change-Id: I3a9d1c22eb0884ca9e37362463fddd0083faf826
Reviewed-on: https://chromium-review.googlesource.com/c/1337462
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>