WebContentsImpl::UpdateTitleForEntry(), if given a blank string
as a title for a file:// url, extracts the filename and uses it
as the new title. That's wrong, as NavigationEntryImpl::
GetTitleForDisplay() already does that, does it *better* than
WebContentsImpl::UpdateTitleForEntry() did, and has tests.

This also removes the |explicit_set| parameter from the
WebContentsObserver::TitleWasSet() method. Its value is of no
use to the vast majority of overriders, and can be computed if
desired.

BUG=96041

Change-Id: Ic08796c2cbadc21c8e08f2eff896fd0f383785df
Reviewed-on: https://chromium-review.googlesource.com/669780


Reviewed-by: Nick Carter <nick@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504570}