Add plugin placeholder and policy support (issue #1708)

- Default plugin loading policy can be specified using the new
  `--plugin-policy=[allow|block|detect]` command-line flag.
- Move CefRequestHandler::OnBeforePluginLoad to
  CefRequestContextHandler and add a new policy argument that
  supports different actions (allow, block, detect, disable) on a
  per-plugin-instance basis.
- Add CefContextMenuHandler::RunContextMenu for providing a custom
  context menu implementation.
- Add CefResourceBundleHandler::GetDataResourceForScale for
  returning scaled resources (issue #1272).
- Add CefResourceBundle for retrieving resources from the resource
  bundle (*.pak) files loaded by CEF during startup or via the
  CefResourceBundleHandler.
- Linux: Fix Debug build IO access warning with CefGetMimeType.
- cef_unittests: Move the refcounting implementation from TestHandler
  to subclasses in order to support interface inheritance from
  subclasses.

tests/unittests/request_handler_unittest.cc

@@ -367,6 +367,8 @@ class NetNotifyTestHandler : public TestHandler {
   TrackCallback got_process_message2_;
   TrackCallback got_before_browse2_will_delay_;
   TrackCallback got_before_browse2_delayed_;
+
+  IMPLEMENT_REFCOUNTING(NetNotifyTestHandler);
 };
 
 // Renderer side.
@@ -897,6 +899,8 @@ class ResourceResponseTest : public TestHandler {
   uint64 main_request_id_;
   uint64 sub_request_id_;
   scoped_ptr<ResourceTest> resource_test_;
+
+  IMPLEMENT_REFCOUNTING(ResourceResponseTest);
 };
 
 }  // namespace
@@ -1050,6 +1054,8 @@ class BeforeResourceLoadTest : public TestHandler {
   TrackCallback got_before_resource_load2_;
   TrackCallback got_load_end_;
   TrackCallback got_load_error_;
+
+  IMPLEMENT_REFCOUNTING(BeforeResourceLoadTest);
 };
 
 }  // namespace