FrameTree::Find only searches relative to local frames.

content/shell/test_runner/test_runner_for_specific_view.cc

 // 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.
 
 #include "content/shell/test_runner/test_runner_for_specific_view.h"
 
 #include <stddef.h>
 #include <limits>
 #include <utility>
 
@@ skipping 661 matching lines @@
 std::string TestRunnerForSpecificView::SelectionAsMarkup() {
   if (!web_view()->MainFrame()->ToWebLocalFrame()) {
     CHECK(false) << "This function cannot be called if the main frame is not a "
                     "local frame.";
   }
   return web_view()->MainFrame()->ToWebLocalFrame()->SelectionAsMarkup().Utf8();
 }
 
 void TestRunnerForSpecificView::SetViewSourceForFrame(const std::string& name,
                                                       bool enabled) {
+  if (!web_view()->MainFrame()->IsWebLocalFrame())

[dcheng, 2017/05/26 18:28:05]

Is this something we'll need to fix at some point? I guess it's probably not a
new problem though...

[Łukasz Anforowicz, 2017/05/26 20:04:05]

Possibly.  For now, let me just do what other similar methods in this source
file do: CHECK(...) that main frame is local.  If any test starts to need OOPIF
support here then the CHECK(...) will hopefully quickly guide them to the
realization that OOPIF support is missing.

+    return;
+
   WebFrame* target_frame =
-      web_view()->FindFrameByName(WebString::FromUTF8(name));
+      web_view()->MainFrame()->ToWebLocalFrame()->FindFrameByName(
+          WebString::FromUTF8(name));
   if (target_frame)
     target_frame->EnableViewSourceMode(enabled);
 }
 
 blink::WebView* TestRunnerForSpecificView::web_view() {
   return web_view_test_proxy_base_->web_view();
 }
 
 WebTestDelegate* TestRunnerForSpecificView::delegate() {
   return web_view_test_proxy_base_->delegate();
 }
 
 }  // namespace test_runner