Make 'web_view_browsertest.cc' BrowserPlugin-free

This CL removes and converts the last remaining BrowserPlugin-based test
to chrome_mime_handler_view_browsertest.cc (the new test uses a
MimeHandlerViewGuest).

Bug: 553069, 330264
Change-Id: I487fbd47ca0c989e746d087c72fe396f9e9ac867
Reviewed-on: https://chromium-review.googlesource.com/1221693
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591115}
5 files changed