This fixes exception creation (by the WebAssembly throw operation) so
that it is not observable by JavaScript. Internal properties are now
stored with symbol names instead of string names, which also prevents
them from being accessed or monkey-patched directly by JavaScript.

R=clemensh@chromium.org
TEST=mjsunit/regress/wasm/regress-8094
BUG=v8:8094

Change-Id: I33cb27f4373114cd4db28d9aef23560093e55242
Reviewed-on: https://chromium-review.googlesource.com/1203951
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55602}