cc: Fix bug where tabs have the wrong scale factor

Device scale factor impacts the draw properties (in particular, the
transforms of the TransformNodes in the property tree). So, mark that
draw properties need to be updated when device scale factor changes.

BUG=567832
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1633043004

Cr-Commit-Position: refs/heads/master@{#371659}

cc/trees/layer_tree_impl.cc

@@ -599,6 +599,7 @@ void LayerTreeImpl::SetDeviceScaleFactor(float device_scale_factor) {
     return;
   device_scale_factor_ = device_scale_factor;
 
+  set_needs_update_draw_properties();
   if (IsActiveTree())
     layer_tree_host_impl_->SetFullRootLayerDamage();
 }

cc/trees/layer_tree_impl_unittest.cc

@@ -2370,5 +2370,13 @@ TEST_F(LayerTreeImplTest, NumLayersSmallTree) {
   EXPECT_EQ(4u, host_impl().active_tree()->NumLayers());
 }
 
+TEST_F(LayerTreeImplTest, DeviceScaleFactorNeedsDrawPropertiesUpdate) {
+  host_impl().active_tree()->SetDeviceScaleFactor(1.f);
+  host_impl().active_tree()->UpdateDrawProperties(false);
+  EXPECT_FALSE(host_impl().active_tree()->needs_update_draw_properties());
+  host_impl().active_tree()->SetDeviceScaleFactor(2.f);
+  EXPECT_TRUE(host_impl().active_tree()->needs_update_draw_properties());
+}
+
 }  // namespace
 }  // namespace cc