Add BlimpDocument, pull out functions in BlimpCompositor.

blimp/client/test/BUILD.gn

 # Copyright 2016 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("test") {
   testonly = true
 
   sources = [
     "contents/mock_blimp_contents_observer.cc",
     "contents/mock_blimp_contents_observer.h",
     "test_blimp_client_context_delegate.cc",
     "test_blimp_client_context_delegate.h",
   ]
 
   deps = [
     "//base",
     "//base/test:test_support",
     "//blimp/client/core",
     "//blimp/client/public:public_headers",
     "//cc:test_support",
     "//google_apis:test_support",
     "//mojo/edk/system",
     "//net",
     "//ui/gfx",
   ]
 
   public_deps = [
     "//blimp/client/test/compositor",
+    "//blimp/client/test/render_widget",
     "//testing/gmock",
   ]
 }