Refactor Framebuffer::invalidateSub.
Refactor Framebuffer::invalidateSub.

Use our new helper methods to simplify this method.

BUG=angle:732

Change-Id: I56b837678d5efbc60de6de4bdb01447ca811e71d
Reviewed-on: https://chromium-review.googlesource.com/213855
Tested-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Shannon Woods <0f3694938fd5703bd548127ee497e4d604509bef@chromium.org>

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