[Ozone-Drm] Fallback silently when main thread runner is not present

ui/ozone/platform/drm/gpu/drm_gpu_platform_support.cc

Index: ui/ozone/platform/drm/gpu/drm_gpu_platform_support.cc
diff --git a/ui/ozone/platform/drm/gpu/drm_gpu_platform_support.cc b/ui/ozone/platform/drm/gpu/drm_gpu_platform_support.cc
index 58dfe35ffc4cd1a28b6bed0d844587cc40b69d93..76d4108ce3307547a78ef30bfdbef13317e8661b 100644
--- a/ui/ozone/platform/drm/gpu/drm_gpu_platform_support.cc
+++ b/ui/ozone/platform/drm/gpu/drm_gpu_platform_support.cc
@@ -37,9 +37,16 @@ class DrmGpuPlatformSupportMessageFilter : public IPC::MessageFilter {
       : window_manager_(window_manager),
         on_filter_added_callback_(on_filter_added_callback),
         main_thread_listener_(main_thread_listener),
-        main_thread_task_runner_(base::ThreadTaskRunnerHandle::Get()),
+        main_thread_task_runner_(0),
         pending_main_thread_operations_(0),
-        cursor_animating_(false) {}
+        cursor_animating_(false) {
+    // A bunch of unit tests and targets don't initialize the message loop, so
+    // the GPU won't have the main thread's handler. In this case, fallback
+    // silently and don't update the cursor related stuff in OnMessageReceived
+    // (which is fine, because most of those targets don't use cursor at all).
+    if (base::ThreadTaskRunnerHandle::IsSet())
+      main_thread_task_runner_ = base::ThreadTaskRunnerHandle::Get();
+  }
 
   void OnFilterAdded(IPC::Sender* sender) override {
     io_thread_task_runner_ = base::ThreadTaskRunnerHandle::Get();
@@ -64,6 +71,11 @@ class DrmGpuPlatformSupportMessageFilter : public IPC::MessageFilter {
     if (!cursor_position_message && !cursor_state_message)
       return false;
 
+    if (!main_thread_task_runner_) {
+      NOTREACHED() << "Cursor implementation fail.";
+      return false;
+    }
+
     bool cursor_was_animating = cursor_animating_;
     UpdateAnimationState(message);
     if (cursor_state_message || pending_main_thread_operations_ ||