Since there are no longer any significant differences in the child
classes of TextureStorageInterface, we can scrap the different
implementations. This involves changing the function names of some
Renderer functions which relied on overloading.

BUG=angle:741

Change-Id: I074797e4ac0bf3ecf8fe4bddf84eaed28b1a2c23
Reviewed-on: https://chromium-review.googlesource.com/218315Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>