This is a reland of 163c1c82
Original change's description:
> [wasm] Gracefully handle malformed custom sections in WebAssembly.Module.customSections().
> 
> R=clemensh@chromium.org
> BUG=chromium:789952
> 
> Change-Id: Ida627fa6cdeacff01a0ec4d20e58281f17528010
> Reviewed-on: https://chromium-review.googlesource.com/800941
> Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
> Commit-Queue: Ben Titzer <titzer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#49767}

Bug: chromium:789952
Change-Id: Ie17629b3fcbf2d5f78c83be2aa2a6b904a61f3ab
Reviewed-on: https://chromium-review.googlesource.com/803575
Commit-Queue: Ben L. Titzer <titzer@google.com>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49796}