Make printing PDF work from inside PDF extension.

chrome/browser/printing/print_view_manager.cc

Index: chrome/browser/printing/print_view_manager.cc
diff --git a/chrome/browser/printing/print_view_manager.cc b/chrome/browser/printing/print_view_manager.cc
index 6afd07b443725b2bb3b55f195554bbb458a5b643..dadead97afbf3135274ded98b3fe7e5d853f4202 100644
--- a/chrome/browser/printing/print_view_manager.cc
+++ b/chrome/browser/printing/print_view_manager.cc
@@ -14,6 +14,7 @@
 #include "chrome/browser/printing/print_preview_dialog_controller.h"
 #include "chrome/browser/ui/webui/print_preview/print_preview_ui.h"
 #include "chrome/common/chrome_content_client.h"
+#include "components/guest_view/browser/guest_view_base.h"
 #include "components/printing/common/print_messages.h"
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/plugin_service.h"
@@ -21,6 +22,7 @@
 #include "content/public/browser/render_process_host.h"
 #include "content/public/browser/web_contents.h"
 #include "content/public/common/webplugininfo.h"
+#include "extensions/common/constants.h"
 #include "printing/features/features.h"
 
 using content::BrowserThread;
@@ -127,6 +129,14 @@ void PrintViewManager::PrintPreviewForWebNode() {
   if (print_preview_state_ != NOT_PREVIEWING)
     return;
   print_preview_state_ = USER_INITIATED_PREVIEW;
+  // If this is a PDF extension loaded inside a MimeHandlerViewGuest then we are
+  // here due to clicking on print button inside the PDF extension. We should
+  // use the main frame of the WebContents for print preview.
+  if (guest_view::GuestViewBase::FromWebContents(web_contents()) &&
+      web_contents()->GetLastCommittedURL().host() ==
+          extension_misc::kPdfExtensionId) {
+    print_preview_rfh_ = web_contents()->GetMainFrame();
+  }
 }
 
 void PrintViewManager::PrintPreviewDone() {