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Prior to this change it was possible for the AcceleratorRegistrarImpl
to outlive the WindowManager and then attempt to use it. This patch
ensures any AcceleratorRegistrarImpl are destroyed when the
WindowManager loses its connection.

BUG=none
TEST=none
R=ben@chromium.org

Review-Url: https://codereview.chromium.org/2105653003
Cr-Commit-Position: refs/heads/master@{#402670}