Update dangling markup mitigations.

Still behind a flag, just updating the checks to look for both `\n` and
`<` rather than just the former. This is in line with the patches up at
https://github.com/whatwg/url/pull/284 and
https://github.com/whatwg/fetch/pull/519.

Intent to Remove: https://groups.google.com/a/chromium.org/d/msg/blink-dev/KaA_YNOlTPk/VmmoV88xBgAJ.

Bug: 680970
Change-Id: Ifda61a0afe1f0e97620acef7dc54b005c6f74840
Reviewed-on: https://chromium-review.googlesource.com/514024
Commit-Queue: Mike West <mkwst@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#474341}
69 files changed