[Sanitizer API] Block non-HTML content.

- Block any non-HTML elements in the Sanitizer.
- Also move <noscript> into the baseline drop list.

Bug: 1203238
Change-Id: I245b52c0d7e07983fb2a8a8ac91243bfbc323631
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2851606


Reviewed-by: Yifan Luo <lyf@chromium.org>
Commit-Queue: Daniel Vogelheim <vogelheim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#876962}