This is primarily needed to test D8 under ASan. ASan installs a signal handler
early in the process startup to show stack traces from crashes. We need to make
sure that if V8 does not handle a signal then the existing handler gets a
chance.

This change only applies when using V8's default signal handler. When
integrating with the embedder's signal handler the behavior is unchanged.

Bug: chromium:771948
Change-Id: Ifd560acf9700ec5f714f009530258fa92c83cabe
Reviewed-on: https://chromium-review.googlesource.com/705823Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48429}