Array buffers can now have an allocation that is larger than the actual
buffer, such as when WebAssembly guard regions are enabled. Embedders
need to know the actual allocation start and length when externalizing
a buffer so they can deallocate it properly.

Bug: chromium:720302, v8:5277
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Ifc184fdd59d77af01c07a64d2c0229ca859a01b0
Reviewed-on: https://chromium-review.googlesource.com/523271
Commit-Queue: Eric Holk <eholk@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45777}