add installPixels

BUG=skia:
R=halcanary@google.com

Review URL: https://codereview.chromium.org/143073008

git-svn-id: http://skia.googlecode.com/svn/trunk@13174 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/gpu/SkGrPixelRef.cpp b/src/gpu/SkGrPixelRef.cpp
index 8a16437..fdf9487 100644
--- a/src/gpu/SkGrPixelRef.cpp
+++ b/src/gpu/SkGrPixelRef.cpp
@@ -178,8 +178,7 @@
         top = 0;
         height = fSurface->height();
     }
-    dst->setConfig(SkBitmap::kARGB_8888_Config, width, height);
-    if (!dst->allocPixels()) {
+    if (!dst->allocPixels(SkImageInfo::MakeN32Premul(width, height))) {
         SkDebugf("SkGrPixelRef::onReadPixels failed to alloc bitmap for result!\n");
         return false;
     }