Fix null pointer dereference in FocusManager::ViewRemoved.

Fix null pointer dereference in FocusManager::ViewRemoved.

Turns out that FocusManager::ViewRemoved could be called
with NULL as the argument. Right now, the code will happily
derefence the null pointer and call a method, which is
undefined behavior in C++.

BUG=617305

[krasin, 2016-06-03 22:47:38]

krasin@google.com changed reviewers:
+ sky@chromium.org

[sky, 2016-06-06 16:30:46]

https://codereview.chromium.org/2036193003/diff/1/ui/views/focus/focus_manage...
File ui/views/focus/focus_manager.cc (right):

https://codereview.chromium.org/2036193003/diff/1/ui/views/focus/focus_manage...
ui/views/focus/focus_manager.cc:520: if (focused_view_ && removed &&
removed->Contains(focused_view_))
This code shouldn't have to check for a null supplied view. This code should
have a DCHECK(removed) and callers shouldn't supply null.

[krasin, 2016-06-08 05:18:37]

Closing this issue in favor of slightly more through fix:
https://codereview.chromium.org/2047083002/
Still not ideal, as I don't fully understand the chain of the events, but the
fix feels future proof: if we ever get this condition again, the DCHECK will
warn us.