Add histograms to measure "unsafe" hostnames.

"Unsafe" hostnames have numeric TLDs but are not IPv4. e.g. "foo.127.1".
When the final two components are numeric, then converting an origin
to a SchemefulSite and back results in a different origin. We want to
get some rough measurement of usage of these before updating the URL
spec to consider them invalid.

This CL adds logging of unsafe hostnames in the following locations:
* URL construction (numbers here aren't terribly meaningful, but not
all URLs makes it to the network layer).
* DNS lookup success/failure.
* File URL request success/failure.

Bug: 1149194
Change-Id: I7d4d4a28461be1c1c7d6a4ca0e4dc92bb1fa332d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2723071
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#860291}

• content/browser/loader/file_url_loader_factory.cc[diff]
• net/dns/host_resolver_manager.cc[diff]
• tools/metrics/histograms/enums.xml[diff]
• tools/metrics/histograms/histograms_xml/net/histograms.xml[diff]
• url/url_canon_host.cc[diff]
• url/url_canon_ip.cc[diff]
• url/url_canon_ip.h[diff]
• url/url_canon_unittest.cc[diff]