Using a little bit more of code repetition allows us to move the
implementation of the mipmaps to a simple virtual interface in
TextureStorage.

BUG=angle:741

Change-Id: I7661d0cc3bdb02b646da0802cba285650d7832b2
Reviewed-on: https://chromium-review.googlesource.com/218314Reviewed-by: Brandon Jones <bajones@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>