Fix viewport computation in Mac UC

cc/output/compositor_frame_metadata.h

Index: cc/output/compositor_frame_metadata.h
diff --git a/cc/output/compositor_frame_metadata.h b/cc/output/compositor_frame_metadata.h
index f21a2e3dfd1f613160b053bb1f56163659ae320a..4beb67830b727b6857670413e16983dec8a1b4e1 100644
--- a/cc/output/compositor_frame_metadata.h
+++ b/cc/output/compositor_frame_metadata.h
@@ -30,7 +30,7 @@ class CC_EXPORT CompositorFrameMetadata {
 
   // These limits can be used together with the scroll/scale fields above to
   // determine if scrolling/scaling in a particular direction is possible.
-  gfx::SizeF viewport_size;
+  gfx::SizeF scrollable_viewport_size;
   gfx::SizeF root_layer_size;
   float min_page_scale_factor;
   float max_page_scale_factor;