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Currently, there is a field on KeyboardController for enqueuing a
container type change. Eventually we want to support invoking a JS
callback to notify the extension when a keyboard mode changes.

This is the first step in that direction. Instead of persisting a
single enum value for the enqueued container behavior, it tracks both
the type and a callback handle. This QueuedContainerBehavior object
will invoke the callback once it goes out of scope.

Change-Id: Iea4e96259314a34d31661342024a23617a9f4b74
Bug: 806189
Reviewed-on: https://chromium-review.googlesource.com/888119
Commit-Queue: Blake O'Hare <blakeo@chromium.org>
Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533197}