Add a step to generate breakpad symbols for content shell on linux

BUG=247431
R=rsesek@chromium.org

Review URL: https://codereview.chromium.org/39693003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230780 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230780