StackDumpSignalHandler is used by EnableInProcessStackDumping, usually on non-
official builds. On Mac it prints the stack and re-raises the signal. On Linux
it prints the stack and calls _exit(1). This means that Linux and Chrome OS
builds won't leave core files or call crash_reporter. This is super-confusing
for developers, since most of us don't run official builds.

Print a warning that the crashing process is calling _exit(1) and that a core
file will not be generated.

BUG=551681
TEST=manual, trigger a CHECK, see the warning printed

Review-Url: https://codereview.chromium.org/2694023002
Cr-Commit-Position: refs/heads/master@{#450192}