ozone/wayland: Allow to run ozone/wayland without wl_seat.

This change removes hard requirement on a wl_seat and allows
to run Ozone/Wayland without that. This is required by 1)
tests that will be run with headless weston 2) systems that
run without any perihperial peripheral devices.

TBR=spang@chromium.org

Bug: 1029241
Change-Id: I9970052b3d6d593591eb93738d0b83218abd1353
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088852Reviewed-by: Maksim Sisov <msisov@igalia.com>
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Cr-Commit-Position: refs/heads/master@{#747153}

ui/ozone/platform/wayland/host/wayland_connection.cc

@@ -83,15 +83,16 @@ bool WaylandConnection::Initialize() {
     LOG(ERROR) << "No wl_shm object";
     return false;
   }
-  if (!seat_) {
-    LOG(ERROR) << "No wl_seat object";
-    return false;
-  }
   if (!shell_v6_ && !shell_) {
     LOG(ERROR) << "No Wayland shell found";
     return false;
   }
 
+  // When we are running tests with weston in headless mode, the seat is not
+  // announced.
+  if (!seat_)
+    LOG(WARNING) << "No wl_seat object. The functionality may suffer.";
+
   return true;
 }
 
@@ -122,13 +123,17 @@ void WaylandConnection::MaybePrepareReadQueue() {
 }
 
 void WaylandConnection::ScheduleFlush() {
-  if (scheduled_flush_)
-    return;
-  DCHECK(base::MessageLoopCurrentForUI::IsSet());
-  base::ThreadTaskRunnerHandle::Get()->PostTask(
-      FROM_HERE,
-      base::BindOnce(&WaylandConnection::Flush, base::Unretained(this)));
-  scheduled_flush_ = true;
+  // When we are in tests, the message loop is set later when the
+  // initialization of the OzonePlatform complete. Thus, just
+  // flush directly. This doesn't happen in normal run.
+  if (!base::MessageLoopCurrentForUI::IsSet()) {
+    Flush();
+  } else if (!scheduled_flush_) {
+    base::ThreadTaskRunnerHandle::Get()->PostTask(
+        FROM_HERE,
+        base::BindOnce(&WaylandConnection::Flush, base::Unretained(this)));
+    scheduled_flush_ = true;
+  }
 }
 
 void WaylandConnection::SetCursorBitmap(const std::vector<SkBitmap>& bitmaps,

ui/ozone/platform/wayland/host/wayland_window.cc

@@ -47,7 +47,11 @@ WaylandWindow* WaylandWindow::FromSurface(wl_surface* surface) {
 void WaylandWindow::UpdateBufferScale(bool update_bounds) {
   DCHECK(connection_->wayland_output_manager());
   const auto* screen = connection_->wayland_output_manager()->wayland_screen();
-  DCHECK(screen);
+
+  // The client might not create screen at all.
+  if (!screen)
+    return;
+
   const auto widget = GetWidget();
 
   int32_t new_scale = 0;