content/gpu: Set up a mojom connection between browser and gpu.

content/public/app/mojo/content_gpu_manifest.json

 {
   "name": "content_gpu",
   "display_name": "Content (GPU process)",
   "interface_provider_specs": {
     "service_manager:connector": {
       "provides": {
         "browser": [
           "IPC::mojom::ChannelBootstrap",
-          "service_manager::mojom::ServiceFactory"
+          "service_manager::mojom::ServiceFactory",
+          "ui::mojom::GpuMain"
         ],
         "service_manager:service_factory": [
           "service_manager::mojom::ServiceFactory"
         ]
       },
       "requires": {
         "content_browser": [ "gpu" ],
         "device": [ "device:power_monitor" ]
       }
     }
   }
 }