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This change improves sanitizer in parse_html_subset.js by using
allow-list of tags and attributes.

Bug: 1086318
Change-Id: I56b94350d49bbb9e218336c9292897ac2e619316
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2222115Reviewed-by: dpapad <dpapad@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Jun Kokatsu <Jun.Kokatsu@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#779556}