services/ui/ to use default LSI

Previously, ParentLocalSurfaceIdAllocator would default to an invalid
LocalSurfaceId. A new LSI had to be explicitly allocated before the
normal behavior could use it.

With ParentLocalSurfaceIdAllocator now default initializing a valid LSI,
the various call sites no longer need to explicitly check for the
uninitialized state.

services/ui/ has some of those call sites.

BUG=837030

Change-Id: I1b82b0133de0ecc3e8fa71f5a40f25762732ccf3
Reviewed-on: https://chromium-review.googlesource.com/1038144
Commit-Queue: Chris Blume <cblume@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585927}

services/ui/ws2/client_root.cc

@@ -83,7 +83,6 @@ void ClientRoot::UpdateLocalSurfaceIdIfNecessary() {
   // LocalSurfaceId is used.
   if (last_surface_size_in_pixels_ != size_in_pixels ||
       !server_window->local_surface_id().has_value() ||
-      !server_window->local_surface_id()->is_valid() ||
       last_device_scale_factor_ != window_->layer()->device_scale_factor()) {
     server_window->set_local_surface_id(
         parent_local_surface_id_allocator_.GenerateId());

services/ui/ws2/window_tree_client_unittest.cc

@@ -1225,7 +1225,7 @@ TEST_F(WindowTreeClientTest, DISABLED_SetWindowBounds) {
   wt_client2_->set_track_root_bounds_changes(true);
 
   viz::ParentLocalSurfaceIdAllocator allocator;
-  viz::LocalSurfaceId local_surface_id = allocator.GenerateId();
+  viz::LocalSurfaceId local_surface_id = allocator.GetCurrentLocalSurfaceId();
   wt1()->SetWindowBounds(10, window_1_1, gfx::Rect(0, 0, 100, 100),
                          local_surface_id);
   ASSERT_TRUE(wt_client1()->WaitForChangeCompleted(10));