Add support for running several dump_syms in parallel

This speeds up generating breakpad symbols for content shell

BUG=247431
R=marja@chromium.org

Review URL: https://codereview.chromium.org/48383002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231303 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231303