[Extensions Page] Fix background page inspectable view url

The background page's web contents is registered with the process manager
before the url is loaded, so we need to use the initial url for this, rather
than the last committed url.

BUG=482887

Review URL: https://codereview.chromium.org/1116153002

Cr-Commit-Position: refs/heads/master@{#327838}
diff --git a/extensions/browser/process_manager.h b/extensions/browser/process_manager.h
index 2b7b64f..f2a1138f 100644
--- a/extensions/browser/process_manager.h
+++ b/extensions/browser/process_manager.h
@@ -89,6 +89,11 @@
   // the extension isn't running or doesn't have a background page.
   ExtensionHost* GetBackgroundHostForExtension(const std::string& extension_id);
 
+  // Returns the ExtensionHost for the given |render_frame_host|, if there is
+  // one.
+  ExtensionHost* GetExtensionHostForRenderFrameHost(
+      content::RenderFrameHost* render_frame_host);
+
   // Returns true if the (lazy) background host for the given extension has
   // already been sent the unload event and is shutting down.
   bool IsBackgroundHostClosing(const std::string& extension_id);