Make readback of alpha channel work for RGBA.

src/gpu/gl/GrGLGpu.h

Index: src/gpu/gl/GrGLGpu.h
diff --git a/src/gpu/gl/GrGLGpu.h b/src/gpu/gl/GrGLGpu.h
index 532a864f0712a70a2a73065c062ea4698cee3aed..559bdcaf9d8cde6c154fef8914745dba94e94eed 100644
--- a/src/gpu/gl/GrGLGpu.h
+++ b/src/gpu/gl/GrGLGpu.h
@@ -200,7 +200,8 @@ private:
                        size_t* indexOffsetInBytes);
 
     // Subclasses should call this to flush the blend state.
-    void flushBlend(const GrXferProcessor::BlendInfo& blendInfo);
+    void flushBlend(const GrXferProcessor::BlendInfo& blendInfo,
+                    const GrSwizzle& shaderOutputSwizzle);
 
     bool hasExtension(const char* ext) const { return fGLContext->hasExtension(ext); }