No change to logic, only docs.

This adds a README file to explain WebView Safe Browsing support, and
documents how to add new Safe Browsing threats.

Bug: 918221, 934597
Test: tools/md_browser/md_browser.py
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I962183b4be247ba6c6fa81493bba5028b1972ad9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1546640Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664651}