With this patch, rather than embedding the JSReceiver address directly
in the WasmToJS wrappers, we put that in a fixed array with global handle
scope and instead embed the location of the handle and the index in the
wrapper. This ensures that the wrapper doesn't need to be patched if the
GC kicks in. This is needed to get the WASM code off the GCed heap.

R=mtrofin@chromium.org

Bug: 
Change-Id: Ie5a77a78cdecec51b04f702c63b8e4285e6a2d8d
Reviewed-on: https://chromium-review.googlesource.com/581682
Commit-Queue: Aseem Garg <aseemgarg@chromium.org>
Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46884}