Actually StringToInt() may be setting to 0 on failure, need to verify.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e44498c3ea4925a4868cad26fcee8fb91cd139cd

commit e44498c3ea4925a4868cad26fcee8fb91cd139cd
Author: eroman <eroman@chromium.org>
Date: Fri Jun 30 00:02:37 2017

Check the return value of base::StringToInt() in
ProxyConfigServiceLinux.

The code was assuming it would set the result to 0 on failure, however
this is not part of the API contract (the actual contract is weirder).

This wouldn't have caused any real bugs, other than some
different interpretations of invalid KDE proxy configurations.

The CL changes some of those edge cases in arbitrary ways (i.e. overflowed integer is now 0 rather than max_int).

BUG= 596573 

Review-Url: https://codereview.chromium.org/2968573002
Cr-Commit-Position: refs/heads/master@{#483553}

[modify] https://crrev.com/e44498c3ea4925a4868cad26fcee8fb91cd139cd/net/proxy/proxy_config_service_linux.cc
[modify] https://crrev.com/e44498c3ea4925a4868cad26fcee8fb91cd139cd/net/proxy/proxy_config_service_linux_unittest.cc