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In ChromeOS, each input method has its own settings, which are currently
stored in the extension local storage. This is not good for several
reasons, so we want to move it to the ChromeOS prefs service:

- Putting it in prefs allows us to eventually sync these settings
  across machines.

- When we eventually deprecate the VK extension, there will be no
  more local storage.

- With the work in 895886, Chrome OS need to access these settings anyway.
  It is much easier for the extension to query Chrome OS than the
  other way around, due to how extension APIs work.

We add a new prefs entry for all the input method settings. Each input
method has a dictionary of key/value pairs to use. For the input method
to get/set these settings, we add private APIs in
chrome.inputMethodPrivate to get/set these values.

Bug: 895886
Change-Id: If1366ead35a8274eadae97c7e4cc37a95526d777
Reviewed-on: https://chromium-review.googlesource.com/c/1343471
Commit-Queue: Darren Shen <shend@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Shu Chen <shuchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611903}