This moves the wrapper classes into their own file. It also inlines
all the various methods for performance.

The inline methods are kept out-of-class to avoid problems with
circular dependencies.

Also adds a few missing valid() checks to the inline methods.

Improves the score of the VBO state change test by 1-2%.

Bug: angleproject:3014
Change-Id: Ia760c3a6905a9a5aacfd05db443b72f34947385e
Reviewed-on: https://chromium-review.googlesource.com/c/1436167
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>