Fixes the implementation of wasm exceptions to use a WasmRuntimeError
object, and set the exception tag value as a property of the
object. This guarantees that an uncaught wasm exception is treated
like all other runtime errors.

Bug: v8:6577
Change-Id: I0ab0130444e745178e86c23b3bc9fc9f385c8d05
Reviewed-on: https://chromium-review.googlesource.com/611124Reviewed-by: Bill Budge <bbudge@chromium.org>
Reviewed-by: Eric Holk <eholk@chromium.org>
Commit-Queue: Karl Schimpf <kschimpf@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47346}