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Before this CL, user's statistics about language detection from the CLD3
library were not accessible to Chrome.

This CL introduces a new keyed service called LanguageModel that
collects such statistics and builds a simple model on top of that. This
allows other parts of Chrome to see top languages in which the user
consumes web content.

A design doc:
https://docs.google.com/a/google.com/document/d/1FBD79CEBUKkMtSJKk3fnF1xnlrXd23z2fHgIGQUFOE8/edit?usp=sharing

BUG=653058

Review-Url: https://codereview.chromium.org/2396783002
Cr-Commit-Position: refs/heads/master@{#423702}