viz: Mark RG_16EXT as not supported as a GpuMemoryBuffer format

https://crrev.com/c/2626100 added RG_16EXT as a viz::ResourceFormat, and
accidentally marked it as a supported GpuMemoryBuffer format too.  This
causes crashes on ChromeOS when playing VP9.2 HDR video.

Bug: 1116101, b/177910735
Change-Id: Iaed4ffefa42bcca4b6bd1885d7e090d9f2782b45
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2639578
Auto-Submit: Sunny Sachanandani <sunnyps@chromium.org>
Commit-Queue: kylechar <kylechar@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845342}

components/viz/common/resources/resource_format_utils.cc

@@ -354,7 +354,6 @@ bool IsGpuMemoryBufferFormatSupported(ResourceFormat format) {
     case BGRA_8888:
     case RED_8:
     case R16_EXT:
-    case RG16_EXT:
     case RGBA_4444:
     case RGBA_8888:
     case RGBA_1010102:
@@ -370,6 +369,7 @@ bool IsGpuMemoryBufferFormatSupported(ResourceFormat format) {
     case LUMINANCE_F16:
     case BGR_565:
     case RG_88:
+    case RG16_EXT:
     case RGBX_8888:
     case BGRX_8888:
     case YVU_420: