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Add a new js_html_template() rule to inline HTML content within JS
files during build time.

Having HTML in JS is a requirement of Polymer3. This GN rule allows
HTML development to still happen within HTML files which eliminates
any inconvenience of that constraint during development.

Moreover, js_html_template() is capable of extracting HTML content from
existing Polymer2 HTML files, such that HTML is not duplicated across
v2 and v3 element implementations during a potential migration.

Bug: 965770
Change-Id: I05fdc1a30968f9793eb6ecb59601afc1c11bfb33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1648811
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667359}