gfx: Make conversions from Size to SizeF be explicit.

cc/input/input_handler.h

Index: cc/input/input_handler.h
diff --git a/cc/input/input_handler.h b/cc/input/input_handler.h
index 51db0137a8f6099f19cfa1b351f489b31b057b05..4696fb5ceca81d7c6cc4678935cc8317292d83bf 100644
--- a/cc/input/input_handler.h
+++ b/cc/input/input_handler.h
@@ -16,6 +16,7 @@ namespace gfx {
 class Point;
 class PointF;
 class ScrollOffset;
+class SizeF;
 class Vector2d;
 class Vector2dF;
 }