add peekPixels to SkCanvas and SkSurface

clone of https://codereview.chromium.org/159723006/

Review URL: https://codereview.chromium.org/161733002

git-svn-id: http://skia.googlecode.com/svn/trunk@13427 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gm/matrixconvolution.cpp b/gm/matrixconvolution.cpp
index 47a3581..b986cc1 100644
--- a/gm/matrixconvolution.cpp
+++ b/gm/matrixconvolution.cpp
@@ -25,8 +25,7 @@
 
     void make_bitmap() {
         fBitmap.allocN32Pixels(80, 80);
-        SkBitmapDevice device(fBitmap);
-        SkCanvas canvas(&device);
+        SkCanvas canvas(fBitmap);
         canvas.clear(0x00000000);
         SkPaint paint;
         paint.setAntiAlias(true);