Remote assistance on Chrome OS Part VII - Clipboard Injection

Remote assistance on Chrome OS Part VII - Clipboard Injection

On Chrome OS, the clipboard is managed by aura instead of the underlying
native platform (e.g. x11, ozone, etc).

This CL introduces a class that
1) Monitors the aura clipboard on the host for changes and notifies the client clipboard.
2) Provides an interface to inject clipboard event into aura.

The public API of this class can be called in any thread as internally it
always posts the call to the |ui_task_runner|.  On ChromeOS, that should
be the UI thread of the browser process.

BUG=424717

Committed: https://crrev.com/7a468734170979bca9e48590267d2d123f0c0f84
Cr-Commit-Position: refs/heads/master@{#303168}

Review URL: https://codereview.chromium.org/704683002

Cr-Commit-Position: refs/heads/master@{#303323}