Make HostPortPair::FromString reject strings with invalid ports.

Make HostPortPair::FromString reject strings with invalid ports.

Previously, HostPortPair would reject (Return an empty HostPortPair)
strings that didn't contain <host>:int, but then just DCHECKed int
wasn't too large to be a valid port.

This is a little weird - in general, methods should either require
input be valid, or be robust enough to handle invalid input.  Since
this method is now also run on command line input, it seems like
handling bad ports makes the most sense, so this CL makes it return
empty HostPortPairs in that case.

BUG=433895

Review URL: https://codereview.chromium.org/736493002

Cr-Commit-Position: refs/heads/master@{#304485}