Change CreateWebUIControllerForURL() to return a unique_ptr.

In WebUIControllerFactory and subclasses.

Change-Id: I93f7d17265bfb77f402eef4bd08566614393b21c
Reviewed-on: https://chromium-review.googlesource.com/1087627
Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565142}