ozone: Introduce ClientPixmap and ClientPixmapFactory for non-GPU processes.

ui/ozone/platform/drm/ozone_platform_gbm.cc

Index: ui/ozone/platform/drm/ozone_platform_gbm.cc
diff --git a/ui/ozone/platform/drm/ozone_platform_gbm.cc b/ui/ozone/platform/drm/ozone_platform_gbm.cc
index 5dfda4d555402f0598df137f0f51059588882818..cf8ca696927fce420cfbf91b3e50176d30b036dd 100644
--- a/ui/ozone/platform/drm/ozone_platform_gbm.cc
+++ b/ui/ozone/platform/drm/ozone_platform_gbm.cc
@@ -149,6 +149,11 @@ class OzonePlatformGbm : public OzonePlatform {
     return make_scoped_ptr(
         new DrmNativeDisplayDelegate(display_manager_.get()));
   }
+  base::FileDescriptor GetVirtualDeviceFd() override {
+    DCHECK(display_manager_);
+    return display_manager_->DupVgemFd();
+  }
+
   void InitializeUI() override {
     device_manager_ = CreateDeviceManager();
     window_manager_.reset(new DrmWindowHostManager());