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When a real IME window is up, AppKit spins a nested runloop that
interacts with the IME window over XPC while we're trying to
invalidate our NSTextInputClient (e.g. because the window has been
deleted). However, this may also cause AppKit to *use* the very
NSTextInputClient that we're trying to invalidate.

Specifically, this seems to happen with phonetic languages such as
Korean and Vietnamese. These use rule-based transforms that do not
commonly pop-up a candidate window. In such cases, a key such as
Enter may simultaneously commit a composition _and_ trigger the omnibox
action, which moves focus away from the omnibox thereby invalidating
the textInputClient_.

-BridgedContentView insertText: was the last hold-out that didn't
have a null check. AttributedSubstringForRangeHelper() and
GetFirstRectForRangeHelper() handle a null client already.

  SystemPreferences -> Keyboard -> Input Sources, then interact with
  the omnibox.

Test: On Mac, add/enable the "2-Set Korean" input method under
Bug: 817097
Change-Id: Ia7d1ebb796b9c98437b563ea405cad785e92ea30
Reviewed-on: https://chromium-review.googlesource.com/1124073Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572396}