crash-reporter: Write out a magic string for Chrome crashes. Also do some much needed code cleanup. BUG=chromium:363660 TEST=emerge platform2 Change-Id: Ica9abfd854e2c77d970851805989c86a6a45fdee Reviewed-on: https://chromium-review.googlesource.com/196764 Commit-Queue: Lei Zhang <thestig@chromium.org> Tested-by: Lei Zhang <thestig@chromium.org> Reviewed-by: Mike Frysinger <vapier@chromium.org>