Improve canonicalization of mailto url path components

The canonicalization of the path component of mailto urls is too lax, leading to
information disclosure and possible command injection attacks against mail
clients. To fix this, we will percent-encode more characters in the path
component of mailto urls, matching other browsers.

BUG=711020
TEST=url_unittests

Review-Url: https://codereview.chromium.org/2817213002
Cr-Commit-Position: refs/heads/master@{#465046}
2 files changed