Only generate breakpad symbols for build artefacts

Generating them for system libraries takes a long time, and they're
unlikely to have symbols anyway.

BUG=247431
R=marja@chromium.org

Review URL: https://codereview.chromium.org/48633002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231321 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231321