On posix, only break into debugger when a debugger is present

That way, breakpad will generate a minidump on CHECK() and related
macros (if breakpad is enabled).

BUG=247431
R=mark@chromium.org

Review URL: https://codereview.chromium.org/48513002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231341 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231341