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This change adds a new language detection feature which is behind its
own feature flag to keep it separate from the existing language
detection support.

While the feature flag is off this change should not be user visible.

The earlier version of language detection only supported static content
- that is page content which was present at initial load.
This new feature supports dynamic content - that is page content loaded
after the initial page load.

R=dmazzoni,akihiroota,aboxhall

Bug: 889370
Change-Id: I98154c3d7a80f3f1a6bbf5bc6b29dc2088e01c41
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1697741
Commit-Queue: Chris Hall <chrishall@chromium.org>
Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732298}