Remove the integrity attribute when writing MHTML

integrity can appear on <link> and <script>. Since all <script> elements
are removed, this CL only affects <link>.

If a stylesheet is loaded, the integrity attribute is removed when writing MHTML.
If the stylesheet didn't load, the <link> element is removed when writing MHTML.

Bug: 826139
Change-Id: I5a80a9d07edd995d736b9b6230a805fed51162a3
Reviewed-on: https://chromium-review.googlesource.com/1110263
Commit-Queue: Dan H <harringtond@chromium.org>
Reviewed-by: Jian Li <jianli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575645}
5 files changed