Add ASTC to list of alpha compressed formats

R=bsalomon@google.com, robertphillips@google.com

Author: krajcevski@google.com

Review URL: https://codereview.chromium.org/412113004
diff --git a/include/gpu/GrTypes.h b/include/gpu/GrTypes.h
index 0a73be9..f04a734 100644
--- a/include/gpu/GrTypes.h
+++ b/include/gpu/GrTypes.h
@@ -407,6 +407,7 @@
     switch (config) {
         case kR11_EAC_GrPixelConfig:
         case kLATC_GrPixelConfig:
+        case kASTC_12x12_GrPixelConfig:
         case kAlpha_8_GrPixelConfig:
             return true;
         default: