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It seems that it's possible that GDK events happen while we're going
to unregister the GDK event handler, and the code must be thread-safe.
(A gpointer |data| in the old code seems pointing to the deleted object.)

This CL removes use of |data| pointer and makes the code thread-safe
without adding any mutex.

BUG=417210

Review URL: https://codereview.chromium.org/610523002

Cr-Commit-Position: refs/heads/master@{#297112}