viz: Correct GLInternalFormat() for RG_88

GL_RG8_EXT is part of GL_EXT_texture_rg which is an ES2 extension, so we
must follow ES2 semantics like GL_R8_EXT i.e. internal format must match
data format. Just something I noticed while doing some related work,
probably doesn't matter in practice if we're not creating RG8 textures
with glTexImage2D, etc.

Bug: 1116101
Change-Id: If6042f2432eec13df6b2d27602d87674e19cc78d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2629665Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Auto-Submit: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843860}

components/viz/common/resources/resource_format_utils.cc

@@ -206,8 +206,6 @@ unsigned int GLInternalFormat(ResourceFormat format) {
   switch (format) {
     case R16_EXT:
       return GL_R16_EXT;
-    case RG_88:
-      return GL_RG8_EXT;
     case RG16_EXT:
       return GL_RG16_EXT;
     case ETC1: