Add BlimpDocument, pull out functions in BlimpCompositor.

blimp/client/core/render_widget/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("render_widget") {
   visibility = [ "//blimp/client/*" ]
 
   sources = [
+    "blimp_document.cc",
+    "blimp_document.h",
+    "blimp_document_manager.cc",
+    "blimp_document_manager.h",
     "render_widget_feature.cc",
     "render_widget_feature.h",
   ]
 
   deps = [
     "//blimp/common",
     "//cc/proto",
     "//net",
   ]
 
   public_deps = [
     "//base",
+    "//blimp/client/core/compositor",
     "//blimp/net",
   ]
 }
 
 source_set("test_support") {
   testonly = true
 
   sources = [
     "mock_render_widget_feature_delegate.cc",
     "mock_render_widget_feature_delegate.h",
   ]
 
   public_deps = [
     ":render_widget",
     "//testing/gmock",
   ]
 }
 
 source_set("unit_tests") {
   visibility = [ "//blimp/client/core:unit_tests" ]
 
   testonly = true
 
   sources = [
+    "blimp_document_manager_unittest.cc",
+    "blimp_document_unittest.cc",
     "render_widget_feature_unittest.cc",
   ]
 
   deps = [
     ":render_widget",
     ":test_support",
     "//base",
+    "//blimp/client/test/compositor",
+    "//blimp/client/test/render_widget",
     "//blimp/common",
     "//blimp/net:test_support",
     "//cc/proto",
+    "//cc/surfaces",
     "//net",
     "//net:test_support",
+    "//skia",
     "//testing/gtest",
   ]
 }