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Adds a new helper function that converts a (pref, prefValue) pair to
a (setting, settingChangeValue) pair that can be used to log metrics.
Uses this helper function to automatically capture setting changes
for all pref based settings. As a first example, implements metrics
for the kKeyboardFunctionKeys on the Device page.

Bug:  1133553

Change-Id: Ia1f0118e27bb7e0f610bfe1d75ba0ef324260f7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2459846
Commit-Queue: Daniel Classon <dclasson@google.com>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816692}