viz: Allow ALPHA_8 res format in skia renderer

Skia ganesh supports kAlpha_8_GrPixelConfig, use it for ALPHA_8 resource format.

Change-Id: I7ecf021e6d65c889ea7a87e9349665426484cf6a
Reviewed-on: https://chromium-review.googlesource.com/743787Reviewed-by: weiliangc <weiliangc@chromium.org>
Commit-Queue: Sohan Jyoti Ghosh <sohan.jyoti@huawei.com>
Cr-Commit-Position: refs/heads/master@{#512755}

components/viz/common/resources/resource_format_utils.cc

@@ -175,6 +175,8 @@ GrPixelConfig ToGrPixelConfig(ResourceFormat format) {
       return kRGBA_4444_GrPixelConfig;
     case RGBA_F16:
       return kRGBA_half_GrPixelConfig;
+    case ALPHA_8:
+      return kAlpha_8_GrPixelConfig;
     default:
       break;
   }