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The goal of this CL is to modify content_browser's IMEDriver so we have access
to ui::InputMethodChromeOS's functionality in mus+ash.

This CL:
- Modifies ui::InputMethodChromeOS so it can call an ack callback after
  processing events.
- Introduces InputMethodBridge to acts as a bridge between
  ui::mojom::InputMethod and ui::InputMethodChromeOS.
- Introduces RemoteTextInputClient to send commands received from
  ui::InputMethodChromeOS to a remote client over mojo IPC.
- Modifies content_browser's IMEDriver implementation to use
  ui::InputMethodChromeOS via InputMethodBridge.
- Some unittests to verify that InputMethodBridge works correctly.

BUG=665981

Review-Url: https://codereview.chromium.org/2557493002
Cr-Commit-Position: refs/heads/master@{#437955}