Add BlimpDocument, pull out functions in BlimpCompositor.

blimp/client/test/render_widget/BUILD.gn

Index: blimp/client/test/render_widget/BUILD.gn
diff --git a/blimp/client/test/render_widget/BUILD.gn b/blimp/client/test/render_widget/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..40d53f3bdd3303fa1efe7d6a811fee7143280cfa
--- /dev/null
+++ b/blimp/client/test/render_widget/BUILD.gn
@@ -0,0 +1,20 @@
+# 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") {
+  testonly = true
+
+  sources = [
+    "mock_render_widget_feature.cc",
+    "mock_render_widget_feature.h",
+  ]
+
+  deps = [
+    "//blimp/client/core/render_widget",
+    "//blimp/client/support/compositor",
+    "//cc/proto",
+    "//net",
+    "//testing/gmock",
+  ]
+}