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Most of the JS-based DeclarativeContentCustomBindings are no longer
necessary with native bindings (only the setIcon handling still is).
Remove the majority of the JS bindings, as well as the CssNativeHandler
class (which was only used by the DeclarativeContent JS bindings).

Bug: 938998
TBR=halliwell@chromium.org (removing unused include)

Change-Id: Ie9bf12f4a050b7b79cd55ef2ff361f19ab913cf3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879947
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709232}