In JS to Wasm wrappers, arguments have to be converted from JavaScript's
representation to Wasm's representation. Because of property accessors, this can
result in JavaScript or even asm.js/Wasm code being run. We were previously
setting this flag before doing the parameter conversions, and if these
conversions triggered a Wasm property getter then we would try to set the flag
twice.

With this change, we wait until after all argument conversions are done to set
the flag.

Bug: chromium:769846

R=bradnelson@chromium.org

Change-Id: Ia4b56df45619dcad69f3750bb33cacfedcaeb5b2
Reviewed-on: https://chromium-review.googlesource.com/693414
Commit-Queue: Brad Nelson <bradnelson@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48244}