GitLab

This change:
- writes to settings each time ChromeVox changes a tts property
- updates from prefs on startup
- observes pref changes and announces when rate, pitch, and volume changes
- adds a new key binding (Search+o, s) which opens the tts settings
- adds a new menu item in the ChromeVox menus opening tts settings

There should be no functional change for ChromeVox commands (e.g. keyboard shortcut to change pitch).

Changes to global tts settings should be reflected in ChromeVox immediately once changed.

Change-Id: Ic701c6f7b4422194ac8a957fbaf4fe0fd60e00af
Reviewed-on: https://chromium-review.googlesource.com/1157173
Commit-Queue: David Tseng <dtseng@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579868}