Serialize prefixes for HTML fragments correctly

Serialize prefixes for HTML fragments correctly as described in the spec:
https://html.spec.whatwg.org/#serialising-html-fragments

Bug: 651762
TEST=external/wpt/html/syntax/serializing-html-fragments/serializing.html

Change-Id: I577841f60eea118ce0039e53f95d970c20566261
Reviewed-on: https://chromium-review.googlesource.com/883862
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Rob Buis <rob.buis@samsung.com>
Cr-Commit-Position: refs/heads/master@{#532068}
2 files changed