Finish cleanup from launching material incognito NTP

Merge IncognitoNewTabPageViewMD into IncognitoNewTabPageView and rename
XML file.

BUG=922690

Change-Id: I21e4c886fdbcc6dc666a985174803b8ad205695b
Reviewed-on: https://chromium-review.googlesource.com/c/1480690
Reviewed-by: Becky Zhou <huayinz@chromium.org>
Commit-Queue: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634293}
5 files changed