services/ui: Inject GpuService instance where needed, instead of singleton.

services/ui/common/gpu_service.cc

Index: services/ui/common/gpu_service.cc
diff --git a/services/ui/common/gpu_service.cc b/services/ui/common/gpu_service.cc
index 113d08682570ce0ae112e5e54271b36302a3be8e..8856b7cca80ce62d00ea763537c22e668da7d231 100644
--- a/services/ui/common/gpu_service.cc
+++ b/services/ui/common/gpu_service.cc
@@ -28,7 +28,6 @@ void PostTask(scoped_refptr<base::SingleThreadTaskRunner> runner,
                    base::Bind(callback, std::move(established_channel_host)));
 }
 
-GpuService* g_gpu_service = nullptr;
 }
 
 GpuService::GpuService(shell::Connector* connector)
@@ -49,24 +48,14 @@ GpuService::GpuService(shell::Connector* connector)
 
 GpuService::~GpuService() {
   DCHECK(IsMainThread());
-  DCHECK_EQ(this, g_gpu_service);
   if (gpu_channel_)
     gpu_channel_->DestroyChannel();
-  g_gpu_service = nullptr;
 }
 
 // static
 std::unique_ptr<GpuService> GpuService::Initialize(
     shell::Connector* connector) {
-  DCHECK(!g_gpu_service);
-  g_gpu_service = new GpuService(connector);
-  return base::WrapUnique(g_gpu_service);
-}
-
-// static
-GpuService* GpuService::GetInstance() {
-  DCHECK(g_gpu_service);
-  return g_gpu_service;
+  return base::WrapUnique(new GpuService(connector));
 }
 
 void GpuService::EstablishGpuChannel(