From asm.js code we might get an empty ArrayBuffer as heap memory. In
this case, both the old memory start and the new memory start will be
nullptr. The size however has to be patched from default_size to 0.

This CL changes code specialization to be able to either patch memory
references, or patch memory sizes or both.

R=titzer@chromium.org, ahaas@chromium.org
BUG=chromium:698587

Change-Id: I4d9d811d75cb83842f23df317e8e7fc02aeb5146
Reviewed-on: https://chromium-review.googlesource.com/450257
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43613}