JavaScriptDialogManager::RunJavaScriptDialog() takes an URL
|origin_url|. Unfortunately, there's two problems here:
- It's not clear what the URL belongs to (the WebContents' top-level
  frame, since the WebContents is also passed in, or the frame that
  is creating the JS dialog).
- It's not an origin at all; it's just an URL.

Rename this to be |alerting_frame_url| to clear up the confusion.

Bug: None

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_site_isolation
Change-Id: I196ef03f87d18b37345fb0028a1cdec4487ec294
Reviewed-on: https://chromium-review.googlesource.com/635943


Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#497808}