The validation for sampler completeness had a lot of duplicated code.
The special cases have been merged into the base class by checking mTarget.

BUG=angle:681

Change-Id: I11d94c1432f8fc70a1edaaf8228bbc43c3c8fff3
Reviewed-on: https://chromium-review.googlesource.com/236932Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>