content: implement unittests backend for Ozone GBM

ui/ozone/platform/cast/ozone_platform_cast.cc

Index: ui/ozone/platform/cast/ozone_platform_cast.cc
diff --git a/ui/ozone/platform/cast/ozone_platform_cast.cc b/ui/ozone/platform/cast/ozone_platform_cast.cc
index 60f7c9a901220a23d2ed3f2662da1cdb6a714211..2b0705561d85f44ccd07fb718f1d5355fc93f9f3 100644
--- a/ui/ozone/platform/cast/ozone_platform_cast.cc
+++ b/ui/ozone/platform/cast/ozone_platform_cast.cc
@@ -80,6 +80,11 @@ class OzonePlatformCast : public OzonePlatform {
         new GpuPlatformSupportCast(surface_factory_.get()));
   }
 
+  void InitializeTest() override {
+    InitializeUI();
+    InitializeGPU();
+  }
+
  private:
   scoped_ptr<CastEglPlatform> egl_platform_;
   scoped_ptr<SurfaceFactoryCast> surface_factory_;