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This patch adds the clear() method to inline & declared StylePropertyMap
Spec: https://drafts.css-houdini.org/css-typed-om-1/#dom-stylepropertymap-clear

Bug: 813653
Change-Id: Ib56f60fa59dc8d241e7fb1a7695fce2bea094026
Reviewed-on: https://chromium-review.googlesource.com/925761
Commit-Queue: Darren Shen <shend@chromium.org>
Reviewed-by: nainar <nainar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537717}