Add the capability to run multiple proxy PAC scripts in parallel.

Refactors SingleThreadedProxyResolver into MultiThreadedProxyResolver.

New threads are created lazily on demand, up to a fixed maximum.

Note that this CL does NOT change the policy in Chrome -- it will continue to use a single thread for proxy resolving, but using the new code to do so.

BUG=11079
Review URL: http://codereview.chromium.org/2822043

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@51924 0039d316-1c4b-4281-b951-d872f2087c98
21 files changed