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Mojo doesn't allow an unrun callback to be removed before the
message pipe is closed. However, Mac spellchecker manages
RequestTextCheckCallbacks as (indirectly) owned by a completion
barrier in SpellingRequest, which lives independently with the
message pipe. Hence, in some shutdown orders, the callback can
be cleared while the message pipe is still open.

This patch fixes the issue by making SpellCheckHost own
all pending SpellingRequests, and SpellingRequest own
RequestTextCheckCallback.

Note: this patch serves as a quick fix to the regressed test
cases. A follow-up patch will clean up the code.

Bug: 814845
Change-Id: I22a08cdce2744b50fe23c0ff48c10d5207fa5f8b
Reviewed-on: https://chromium-review.googlesource.com/946848
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541624}