mus/aura: Make sure the root window always has a valid LocalSurfaceId.

When running with mus, and mus is not hosting viz, aura has to generate
the LocalSurfaceId. It needs to make sure the root window always has a
valid LocalSurfaceId, because that's what is fed into ui::Compositor,
and without a valid id, it never shows anything on screen.

BUG=786453

Change-Id: Ie075246937f7e607afdc945ea1cea0b021595e50
Reviewed-on: https://chromium-review.googlesource.com/807886Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521762}

ui/aura/mus/window_port_mus.cc

@@ -17,6 +17,7 @@
 #include "ui/aura/window.h"
 #include "ui/aura/window_delegate.h"
 #include "ui/aura/window_observer.h"
+#include "ui/aura/window_tree_host.h"
 #include "ui/base/class_property.h"
 #include "ui/base/ui_base_switches_util.h"
 #include "ui/display/display.h"
@@ -401,7 +402,12 @@ void WindowPortMus::AllocateLocalSurfaceId() {
 }
 
 const viz::LocalSurfaceId& WindowPortMus::GetLocalSurfaceId() {
-  return local_surface_id_;
+  if (switches::IsMusHostingViz())
+    return local_surface_id_;
+  if (window_->GetRootWindow() != window_)
+    return local_surface_id_;
+  return GetOrAllocateLocalSurfaceId(
+      window_->GetHost()->GetBoundsInPixels().size());
 }
 
 std::unique_ptr<WindowMusChangeData>

ui/aura/window_unittest.cc

@@ -35,6 +35,7 @@
 #include "ui/aura/window_tree_host.h"
 #include "ui/base/class_property.h"
 #include "ui/base/hit_test.h"
+#include "ui/base/ui_base_switches_util.h"
 #include "ui/compositor/layer.h"
 #include "ui/compositor/layer_animation_observer.h"
 #include "ui/compositor/layer_animator.h"
@@ -420,6 +421,11 @@ TEST_P(WindowTest, ContainsMouse) {
   EXPECT_FALSE(w_test_api.ContainsMouse());
 }
 
+// Tests that the root window gets a valid LocalSurfaceId.
+TEST_P(WindowTest, RootWindowHasValidLocalSurfaceId) {
+  EXPECT_TRUE(root_window()->GetLocalSurfaceId().is_valid());
+}
+
 // Test Window::ConvertPointToWindow() with transform to root_window.
 TEST_P(WindowTest, MoveCursorToWithTransformRootWindow) {
   gfx::Transform transform;