crash-reporter: Make use of list_proxies to send crashes via a proxy when appropriate

BUG=chromium-os:6828
TEST=Manually ran crash_sender to make sure it would run curl with appropriate --proxy option.  Manually ran curl using proxies from "http://proxyconfig.corp.google.com/wpad.dat".  Ran a modified crash_sender manually to make sure it passed the appropriate "--proxy" arguments correctly.

Review URL: http://codereview.chromium.org/6802003

Change-Id: Ie1414fca1a7dbc265d8aad0eea32a736a5d30d54
1 file changed