GitLab

- Add MRDialogControllerViewsImpl stub
- Rename MRDialogControllerImpl to MRDialogControllerWebUIImpl
- Factor out parts that are common across WebUI and Views into
  MRDialogControllerImplBase
- MediaRouterDialogControllerImplBase::GetOrCreateForWebContents() is
  implemented in two places: mrdc_views_impl.cc for builds with Views,
  mrdc_webui_impl.cc for builds without.

Bug: 826091
Change-Id: I276d46ba870b837c5b35f48a4921a3df356aa565
Reviewed-on: https://chromium-review.googlesource.com/990572Reviewed-by: Derek Cheng <imcheng@chromium.org>
Reviewed-by: Adam Parker <amp@chromium.org>
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548290}