Fix user agent overridding with PlzNavigate.

content/browser/frame_host/navigator_delegate.h

Index: content/browser/frame_host/navigator_delegate.h
diff --git a/content/browser/frame_host/navigator_delegate.h b/content/browser/frame_host/navigator_delegate.h
index c3db7d188332668261784b124efbf04e29fe140a..ad1aba9bc207417e6e99dd5bb4d263595e92488e 100644
--- a/content/browser/frame_host/navigator_delegate.h
+++ b/content/browser/frame_host/navigator_delegate.h
@@ -105,6 +105,9 @@ class CONTENT_EXPORT NavigatorDelegate {
   // preserved in the omnibox.  Defaults to false.
   virtual bool ShouldPreserveAbortedURLs();
 
+  // Returns the overriden user agent string if it's set.
+  virtual const std::string& GetNavigationUserAgentOverride();

[nasko, 2017/03/13 18:47:12]

nit: The "Navigation" part of the name doesn't really add much meaning to the
method. We will use the same user agent override for subresources too, so making
it shorter and easier to read is a win IMHO.

[jam, 2017/03/13 19:32:40]

yeah I did it this way because WebContents, which implements, already has a
GetUserAgentOverride. I know I could leave it as is, and the one override will
work for both interfaces, but I find that it's more confusing that way for
readability. wdyt?

[nasko, 2017/03/13 20:35:41]

We have a few cases where the same pure virtual method exist on this delegate
interface and on WebContentsImpl. As long as the function is exactly the same in
both cases, we keep them exactly the same. An example of that is the OpenURL
call above. This case looks identical, so I personally don't find it more
confusing. It is one method on the same object, reachable through different
interfaces.

[jam, 2017/03/13 20:38:25]

ah, if there's already precedent with this interface I'll do the same, thanks.

Personally I didn't find this confusing but I got feedback in reviews before
that it it, so happy to go with this approach :)

+
   // A RenderFrameHost in the specified |frame_tree_node| started loading a new
   // document. This correponds to Blink's notion of the throbber starting.
   // |to_different_document| will be true unless the load is a fragment