Allow SIGABRT to abort the program.

SIGABRT can be generated internally, usually by calling abort(),
or externally by another process.  When the signal is generated
by the kernel, info->si_pid is 0 and the signal is treated in the
same way as an exception (SIGSEGV, etc.), but the assumption
that the exception happens again upon return from the handler
is wrong, so we must have a special case for this.

Original CL: https://breakpad.appspot.com/734002/

BUG=chromium:303075
TEST=tested with Alt-VolumeUp-X on Chrome OS
A=semenzato@chromium.org
R=semenzato@google.com

Review URL: https://breakpad.appspot.com/754002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1233 4c0a9323-5329-0410-9bdc-e9ce6186880e