switch GatherPixelRefs to use SkBaseDevice instead of SkBitmapDevice

src/core/SkBitmapDevice.cpp

Index: src/core/SkBitmapDevice.cpp
diff --git a/src/core/SkBitmapDevice.cpp b/src/core/SkBitmapDevice.cpp
index 637048742a1a12ddfec1fbdb2bd576618aa71bc1..030331ac51820b4207b2a1ef01aed9414c1773e0 100644
--- a/src/core/SkBitmapDevice.cpp
+++ b/src/core/SkBitmapDevice.cpp
@@ -77,14 +77,6 @@ void SkBitmapDevice::unlockPixels() {
     }
 }
 
-void SkBitmapDevice::getGlobalBounds(SkIRect* bounds) const {
-    if (NULL != bounds) {
-        const SkIPoint& origin = this->getOrigin();
-        bounds->setXYWH(origin.x(), origin.y(),
-                        fBitmap.width(), fBitmap.height());
-    }
-}
-
 void SkBitmapDevice::clear(SkColor color) {
     fBitmap.eraseColor(color);
 }