Add GenericSharedMemoryId and use w/ GpuMemoryBuffer

cc/test/test_gpu_memory_buffer_manager.cc

Index: cc/test/test_gpu_memory_buffer_manager.cc
diff --git a/cc/test/test_gpu_memory_buffer_manager.cc b/cc/test/test_gpu_memory_buffer_manager.cc
index 6648823b29cd546dee0142ad8d289c4497dfedf5..8d56a97c67e720863f11422b7fb47975eff03d66 100644
--- a/cc/test/test_gpu_memory_buffer_manager.cc
+++ b/cc/test/test_gpu_memory_buffer_manager.cc
@@ -123,7 +123,7 @@ class GpuMemoryBufferImpl : public gfx::GpuMemoryBuffer {
   }
   gfx::GpuMemoryBufferId GetId() const override {
     NOTREACHED();
-    return 0;
+    return gfx::GpuMemoryBufferId(0);
   }
   gfx::GpuMemoryBufferHandle GetHandle() const override {
     gfx::GpuMemoryBufferHandle handle;