gfx: Make conversions from Size to SizeF be explicit.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 36e7bdd879eb9fe90e622b4db1611502f8f6f1d4..5b96993e159e0e4be502d044a6bc3e797803ab44 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -452,7 +452,7 @@ void LayerTreeHostImpl::StartPageScaleAnimation(
   gfx::ScrollOffset scroll_total = active_tree_->TotalScrollOffset();
   gfx::SizeF scaled_scrollable_size = active_tree_->ScrollableSize();
   gfx::SizeF viewport_size =
-      active_tree_->InnerViewportContainerLayer()->bounds();
+      gfx::SizeF(active_tree_->InnerViewportContainerLayer()->bounds());
 
   // Easing constants experimentally determined.
   scoped_ptr<TimingFunction> timing_function =
@@ -2247,7 +2247,7 @@ void LayerTreeHostImpl::SetViewportSize(const gfx::Size& device_viewport_size) {
   SetFullRootLayerDamage();
   active_tree_->set_needs_update_draw_properties();
   active_tree_->property_trees()->clip_tree.SetViewportClip(
-      gfx::RectF(device_viewport_size));
+      gfx::RectF(gfx::SizeF(device_viewport_size)));
 }
 
 void LayerTreeHostImpl::SetDeviceScaleFactor(float device_scale_factor) {