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When show emoji/voice/handwriting vk with opt-in IME menu with virtual
keyboard disabled, the virtual keyboard will be forced shown once.
Thus, after the virtual keyboard hidden, the menu will disable the vk
to make sure it won't automatically brown.
The menu listens on KeyboardController::OnKeyboardHidden() event to
disable the vk, where SIGSEGV will cause.

Fix the issue by posting the task of disabling virtual keyboard.

BUG=721673
TEST=Verified on local build

Review-Url: https://codereview.chromium.org/2884873002
Cr-Commit-Position: refs/heads/master@{#472300}