viz: Convert a sync api in ServerGpuMemoryBufferManager into async.

components/viz/common/BUILD.gn

 # Copyright 2017 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 source_set("common") {
   sources = [
     "server_gpu_memory_buffer_manager.cc",
     "server_gpu_memory_buffer_manager.h",
   ]
 
   deps = [
     "//gpu/ipc/client",
     "//gpu/ipc/common",
     "//services/ui/gpu/interfaces",
   ]
 
   public_deps = [
     "//gpu/command_buffer/client",
     "//gpu/ipc/host",
     "//ui/gfx",
   ]
 }
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "server_gpu_memory_buffer_manager_unittest.cc",
+  ]
+
+  deps = [
+    ":common",
+    "//base/test:test_support",
+    "//services/ui/gpu/interfaces",
+    "//testing/gtest",
+  ]
+}