Make SkGpuDevice know its alpha type

src/gpu/GrSurface.cpp

Index: src/gpu/GrSurface.cpp
diff --git a/src/gpu/GrSurface.cpp b/src/gpu/GrSurface.cpp
index c052a235f07e023d8b70f878305400288f406274..b304ccb1ddbff9d2d9de3c2f010258bcd6ade72b 100644
--- a/src/gpu/GrSurface.cpp
+++ b/src/gpu/GrSurface.cpp
@@ -41,13 +41,13 @@ bool GrSurface::readPixels(int left, int top, int width, int height,
     return false;
 }
 
-SkImageInfo GrSurface::info() const {
+SkImageInfo GrSurface::info(SkAlphaType alphaType) const {
     SkColorType colorType;
     SkColorProfileType profileType;
     if (!GrPixelConfig2ColorAndProfileType(this->config(), &colorType, &profileType)) {
         sk_throw();
     }
-    return SkImageInfo::Make(this->width(), this->height(), colorType, kPremul_SkAlphaType,
+    return SkImageInfo::Make(this->width(), this->height(), colorType, alphaType,
                              profileType);
 }