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Very basic skeleton of the interface with search box and list of
emoji. Currently draws emoji into groups from the emoji_ordering.json
file. There is also a row of icons (currently not rendering).

Some changes to BUILD.gn have been made to build the components
using Polymer. This allows us to write template in html and have them
imported into the js component code (see `__html_template__` placeholders)

Bug: 1152237
Change-Id: Ie7f803ba8c2f45d136e81ae2f6b0dd95d616fd27
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2591988
Commit-Queue: Kenton Lam <kentonlam@google.com>
Reviewed-by: Keith Lee <keithlee@chromium.org>
Reviewed-by: John Palmer <jopalmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837808}