switch GatherPixelRefs to use SkBaseDevice instead of SkBitmapDevice

include/core/SkDevice.h

Index: include/core/SkDevice.h
diff --git a/include/core/SkDevice.h b/include/core/SkDevice.h
index 22a56d007de034749791b33d7d1e82cd462cf78e..9801c43e2ba7c7a4ac64de6fab230c079d6cdf15 100644
--- a/include/core/SkDevice.h
+++ b/include/core/SkDevice.h
@@ -83,7 +83,12 @@ public:
      *  canvas. The root device will have its top-left at 0,0, but other devices
      *  such as those associated with saveLayer may have a non-zero origin.
      */
-    virtual void getGlobalBounds(SkIRect* bounds) const = 0;
+    void getGlobalBounds(SkIRect* bounds) const {
+        SkASSERT(bounds);
+        const SkIPoint& origin = this->getOrigin();
+        bounds->setXYWH(origin.x(), origin.y(), this->width(), this->height());
+    }
+    
 
     /** Returns true if the device's bitmap's config treats every pixel as
         implicitly opaque.