Make SkGpuDevice know its alpha type

Make SkImage_Gpu snapshots opaque if surface is opaque.

BUG=skia:3965

Review URL: https://codereview.chromium.org/1205643002
diff --git a/src/gpu/GrSurface.cpp b/src/gpu/GrSurface.cpp
index c052a23..b304ccb 100644
--- a/src/gpu/GrSurface.cpp
+++ b/src/gpu/GrSurface.cpp
@@ -41,13 +41,13 @@
     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);
 }