Added environment variable to set proxy server from a file in the stateful partition.
Added environment variable to set proxy server from a file in the stateful partition.

BUG=none
TEST=Verified proxy set correctly with "about:net-internals" when running image natively on a machine.

Review URL: http://codereview.chromium.org/2989001

chadversary/chromiumos.platform.login_manager,joshfire/chromiumos-platform-login_manager,joshfire/chromiumos-platform-login_manager,chadversary/chromiumos.platform.login_manager,joshfire/chromiumos-platform-login_manager,chadversary/chromiumos.platform.login_manager