add peekPixels to canvas

src/core/SkDevice.cpp

Index: src/core/SkDevice.cpp
diff --git a/src/core/SkDevice.cpp b/src/core/SkDevice.cpp
index 9ce8b6b5df2e2a002bbd7fee9d5836df557c244d..4bf8516fd7a1dca231d5f97675e0fc71d5a4b0bc 100644
--- a/src/core/SkDevice.cpp
+++ b/src/core/SkDevice.cpp
@@ -65,6 +65,16 @@ SkMetaData& SkBaseDevice::getMetaData() {
     return *fMetaData;
 }
 
+// TODO: when we can remove width/height/config from SkBaseDevice, then other
+// classes should explicitly override this guy.
+SkImageInfo SkBaseDevice::imageInfo() const {
+    return SkImageInfo::Make(this->width(),
+                             this->height(),
+                             SkBitmapConfigToColorType(this->config()),
+                             this->isOpaque() ? kOpaque_SkAlphaType :
+                                                kPremul_SkAlphaType);
+}
+
 const SkBitmap& SkBaseDevice::accessBitmap(bool changePixels) {
     const SkBitmap& bitmap = this->onAccessBitmap();
     if (changePixels) {
@@ -117,3 +127,5 @@ bool SkBaseDevice::readPixels(SkBitmap* bitmap, int x, int y,
 }
 
 SkSurface* SkBaseDevice::newSurface(const SkImageInfo&) { return NULL; }
+
+const void* SkBaseDevice::peekPixels(SkImageInfo*, size_t*) { return NULL; }