There were two issues with the current implementation:
 * The GetDimensions function already takes into account the base level of the
   SRV.
 * The GetDimensions function returns doesn't return valid sizes for levels
   that don't exist in the SRV.  Instead, manually do the lod offset.

BUG=angleproject:931
BUG=angleproject:1316

TEST=dEQP-GLES3.functional.shaders.texture_functions.texturesize.sampler2d_fixed_vertex

Change-Id: I63259b563a42b93b73949e0ef7ac412099a42f13
Reviewed-on: https://chromium-review.googlesource.com/376099Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>