Add support for half float alpha textures.

include/gpu/GrColor.h

Index: include/gpu/GrColor.h
diff --git a/include/gpu/GrColor.h b/include/gpu/GrColor.h
index f862ecc1d069c3bffaecc17528c5a207b15eebd8..98bdb228405d45bc7ce4185976f299c57aee85c6 100644
--- a/include/gpu/GrColor.h
+++ b/include/gpu/GrColor.h
@@ -168,6 +168,7 @@ static inline uint32_t GrPixelConfigComponentMask(GrPixelConfig config) {
         kA_GrColorComponentFlag,        // kR11_EAC_GrPixelConfig
         kRGBA_GrColorComponentFlags,    // kASTC_12x12_GrPixelConfig
         kRGBA_GrColorComponentFlags,    // kRGBA_float_GrPixelConfig
+        kA_GrColorComponentFlag,        // kAlpha_16_GrPixelConfig
     };
     return kFlags[config];
 
@@ -183,6 +184,7 @@ static inline uint32_t GrPixelConfigComponentMask(GrPixelConfig config) {
     GR_STATIC_ASSERT(9  == kR11_EAC_GrPixelConfig);
     GR_STATIC_ASSERT(10 == kASTC_12x12_GrPixelConfig);
     GR_STATIC_ASSERT(11 == kRGBA_float_GrPixelConfig);
+    GR_STATIC_ASSERT(12 == kAlpha_half_GrPixelConfig);
     GR_STATIC_ASSERT(SK_ARRAY_COUNT(kFlags) == kGrPixelConfigCnt);
 }