Fix post-message support for MimeHandlerViewGuest, add test.

The CL https://crrev.com/737514 fixes an issue with the PDF viewer
extension not loading if another extension injects an additional iframe
into the top-level HTML document for the PDF.

Not caught in the bug description is the fact that PostMessaging support
is also broken; that was discovered (and fixed) while writing the
accompanying test.

Bug: 1046795
Change-Id: I650518162e24e5d961e45b471bfb1518e9b7531c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2037343
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738361}

chrome/browser/pdf/pdf_extension_test.cc

@@ -2310,6 +2310,22 @@ IN_PROC_BROWSER_TEST_F(PDFExtensionTest, DocumentWriteIntoNewPopup) {
   ASSERT_TRUE(pdf_extension_test_util::EnsurePDFHasLoaded(popup));
 }
 
+// Tests that the PDF extension loads in the presence of an extension that, on
+// the completion of document loading, adds an <iframe> to the body element.
+// https://bugs.chromium.org/p/chromium/issues/detail?id=1046795
+IN_PROC_BROWSER_TEST_F(PDFExtensionTest,
+                       PdfLoadsWithExtensionThatInjectsFrame) {
+  // Load the test extension.
+  const extensions::Extension* test_extension = LoadExtension(
+      GetTestResourcesParentDir().AppendASCII("extension_injects_iframe"));
+  ASSERT_TRUE(test_extension);
+
+  // Load the PDF. The call to LoadPdf() will return false if the pdf extension
+  // fails to load.
+  GURL test_pdf_url(embedded_test_server()->GetURL("/pdf/test.pdf"));
+  ASSERT_TRUE(LoadPdf(test_pdf_url));
+}
+
 // This test suite does a simple text-extraction based on the accessibility
 // internals, breaking lines & paragraphs where appropriate.  Unlike
 // TreeDumpTests, this allows us to verify the kNextOnLine and kPreviousOnLine

chrome/test/data/pdf/extension_injects_iframe/inject.js

+// Copyright 2020 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.
+
+document.body.appendChild(document.createElement("iframe"));

chrome/test/data/pdf/extension_injects_iframe/manifest.json

+{
+    "content_scripts": [ {
+        "js": ["inject.js"],
+        "matches": ["<all_urls>"],
+        "run_at": "document_end"
+    }],
+    "description": "Test extension that breaks displaying of PDF files in Chromium.",
+    "manifest_version": 2,
+    "name": "Break PDF Display",
+    "permissions": [ "<all_urls>"],
+    "version": "0.1"
+}

extensions/renderer/guest_view/mime_handler_view/mime_handler_view_container_manager.cc

@@ -313,7 +313,10 @@ blink::WebLocalFrame* MimeHandlerViewContainerManager::GetSourceFrame() {
 }
 
 blink::WebFrame* MimeHandlerViewContainerManager::GetTargetFrame() {
-  return GetSourceFrame()->FirstChild();
+  // Search for the frame using the 'name' attribute, since if an extension
+  // injects other frames into the embedder, there will be more than one frame.
+  return GetSourceFrame()->FindFrameByName(
+      blink::WebString::FromUTF8(internal_id_));
 }
 
 bool MimeHandlerViewContainerManager::IsEmbedded() const {