This is a reland of ee4fe896
Original change's description:
> [wasm] trap handlers: fall back on old signal handler
> 
> 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/705823
> Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
> Commit-Queue: Eric Holk <eholk@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#48429}

Bug: chromium:771948
Change-Id: Ide307091c432fd933c48f89c51851b8dce44dd30
Reviewed-on: https://chromium-review.googlesource.com/710114Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48435}