Make readback of alpha channel work for RGBA.

src/gpu/GrProgramDesc.h

Index: src/gpu/GrProgramDesc.h
diff --git a/src/gpu/GrProgramDesc.h b/src/gpu/GrProgramDesc.h
index 13dd14935d8dff2992dcfebbeb94ab6a7f1bd0f4..9585e058b4a2029b5471e61c61f7ba38205cc9bc 100644
--- a/src/gpu/GrProgramDesc.h
+++ b/src/gpu/GrProgramDesc.h
@@ -70,15 +70,16 @@ public:
     }
 
     struct KeyHeader {
-        uint8_t                     fFragPosKey;   // set by GrGLShaderBuilder if there are
-                                                   // effects that read the fragment position.
-                                                   // Otherwise, 0.
+        // Set by GrGLShaderBuilder if there are effects that read the fragment position.
+        // Otherwise, 0.
+        uint8_t                     fFragPosKey;
+        // Set to uniquely idenitify any swizzling of the shader's output color(s).
+        uint8_t                     fOutputSwizzle;
         uint8_t                     fSnapVerticesToPixelCenters;
         int8_t                      fColorEffectCnt;
         int8_t                      fCoverageEffectCnt;
         uint8_t                     fIgnoresCoverage;
     };
-    GR_STATIC_ASSERT(sizeof(KeyHeader) == 5);
 
     int numColorEffects() const {
         return this->header().fColorEffectCnt;