use bitmap's fRowBytes, as the pixelref's is not (yet) reliable

should fix linux browser_tests for DEPS roll

BUG=
R=robertphillips@google.com

Review URL: https://codereview.chromium.org/132273004

git-svn-id: http://skia.googlecode.com/svn/trunk@13002 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/core/SkBitmap.cpp b/src/core/SkBitmap.cpp
index cb8ac72..f950e28 100644
--- a/src/core/SkBitmap.cpp
+++ b/src/core/SkBitmap.cpp
@@ -341,7 +341,7 @@
             void* p = fPixelRef->pixels();
             if (NULL != p) {
                 p = (char*)p
-                    + fPixelRef->rowBytes() * fPixelRefOrigin.fY
+                    + fPixelRefOrigin.fY * fRowBytes
                     + fPixelRefOrigin.fX * fBytesPerPixel;
             }
             fPixels = p;