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This change introduces ScriptTracker, which keeps track of the
available scripts, runnable scripts, scripts that are run and scripts
that were run and exposes these pieces of information through a simple
interface to the controller.

The tracker isn't bound to the controller yet. The controller is
expected to:
 - call SetScripts and CheckScripts at the right time
 - listen to changes to the set of runnable script, fetch them
   using runnable_scripts() and make them available for selection
 - call ExecuteScript with whatever script was selected

Change-Id: Ice3d2c81e523449b17db2ba8e7735e55ed837ea6
Bug: 806868
Reviewed-on: https://chromium-review.googlesource.com/1193823
Commit-Queue: Stephane Zermatten <szermatt@chromium.org>
Reviewed-by: Ganggui Tang <gogerald@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588821}