Make scaled_scroll_bounds floaat to avoid truncating intermediate results.

Minor refactor to avoid the possibility of accumulating error through
truncation in LayerImpl::MaxScrollOffset().

Review URL: https://codereview.chromium.org/214893005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@260113 0039d316-1c4b-4281-b951-d872f2087c98

cc/layers/layer_impl.cc

@@ -1156,7 +1156,7 @@ gfx::Vector2d LayerImpl::MaxScrollOffset() const {
   DCHECK(this != layer_tree_impl()->InnerViewportScrollLayer() ||
          IsContainerForFixedPositionLayers());
 
-  gfx::Size scaled_scroll_bounds(bounds());
+  gfx::SizeF scaled_scroll_bounds(bounds());
 
   float scale_factor = 1.f;
   for (LayerImpl const* current_layer = this;