Exit SkBitmap::scrollRect() early if width <= 0

As suggested in email from saintlou@google.com
Review URL: http://codereview.appspot.com/4806047

git-svn-id: http://skia.googlecode.com/svn/trunk@1948 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gm/gm.h b/gm/gm.h
index ab92ff6..ce10ec7 100644
--- a/gm/gm.h
+++ b/gm/gm.h
@@ -1,7 +1,9 @@
 #ifndef skiagm_DEFINED
 #define skiagm_DEFINED
 
+#include "SkBitmap.h"
 #include "SkCanvas.h"
+#include "SkDevice.h"
 #include "SkPaint.h"
 #include "SkRefCnt.h"
 #include "SkSize.h"