Manually compute the mipmap size for the textureSize builtin. 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/376099 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Zhenyao Mo <zmo@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>