Add a JavaScript dialog tab helper.

extensions/browser/guest_view/mime_handler_view/mime_handler_view_guest.cc

Index: extensions/browser/guest_view/mime_handler_view/mime_handler_view_guest.cc
diff --git a/extensions/browser/guest_view/mime_handler_view/mime_handler_view_guest.cc b/extensions/browser/guest_view/mime_handler_view/mime_handler_view_guest.cc
index a58e021b4ba8fe95482c2d568067581fc057d036..ec912c598cff72767c0e9055bffb94bdf2f504c3 100644
--- a/extensions/browser/guest_view/mime_handler_view/mime_handler_view_guest.cc
+++ b/extensions/browser/guest_view/mime_handler_view/mime_handler_view_guest.cc
@@ -214,7 +214,7 @@ content::JavaScriptDialogManager*
 MimeHandlerViewGuest::GetJavaScriptDialogManager(
     WebContents* source) {
   return owner_web_contents()->GetDelegate()->GetJavaScriptDialogManager(
-      web_contents());
+      owner_web_contents());

[raymes, 2016/09/01 01:28:30]

Could you explain why you've changed this? Thanks!

[Avi (use Gerrit), 2016/09/01 01:38:02]

Rather than having a single manager globally, I want to have the ability to have
the dialog code in a tab helper, living on the tab. And since the Browser is the
delegate for all of the tabs, the only way it knows which tab's tab helper to
use is that parameter.

Currently, the delegate gets the call "what's the dialog manager for this
WebContents that you're not delegate for?" With a global manager, that doesn't
matter, but with per-tab managers, that becomes a problem.

Once we have the manager, the guest WebContents can pass its pointer to the
manager, but to get the manager itself the guest WebContents has to pretend that
it's its parent, because the delegate doesn't know that the guest WebContents
exists.

[paulmeyer, 2016/09/01 15:45:53]

Nit: Could you please add a short comment here to this effect? (particularly
about the guest pretending to be its parent and why)

 }
 
 bool MimeHandlerViewGuest::SaveFrame(const GURL& url,