add peekPixels to canvas

src/core/SkBitmapDevice.cpp

Index: src/core/SkBitmapDevice.cpp
diff --git a/src/core/SkBitmapDevice.cpp b/src/core/SkBitmapDevice.cpp
index 9dde6e172ec1077fabe5261dea1a5e5ceeefce43..fcc3069d5f94789ec4e165a5b5bd7dfc9d1be63b 100644
--- a/src/core/SkBitmapDevice.cpp
+++ b/src/core/SkBitmapDevice.cpp
@@ -386,6 +386,16 @@ SkSurface* SkBitmapDevice::newSurface(const SkImageInfo& info) {
     return SkSurface::NewRaster(info);
 }
 
+const void* SkBitmapDevice::peekPixels(SkImageInfo* info, size_t* rowBytes) {
+    if (fBitmap.getPixels() && fBitmap.asImageInfo(info)) {
+        if (rowBytes) {
+            *rowBytes = fBitmap.rowBytes();
+        }
+        return fBitmap.getPixels();
+    }
+    return NULL;
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 
 bool SkBitmapDevice::filterTextFlags(const SkPaint& paint, TextFlags* flags) {