- Currently if GrowMemory is called with pages = 0, an attempt is made to
   unregister the ArrayBuffer even if it is external. Cleanup so all Detaching
   of ArrayBuffer is centralized to one method, and can only be called fromJS.
 - Gate creating WeakHandles to the memory on the buffer having guard pages
   enabled. Currently creating a WeakHandle is gated only on if the buffer
   is_external true. If a buffer is marked is_external = true to begin with,
   the WeakHandle is created and the Finalizer is run causing the program to
   crash.

BUG=chromium:717647

Review-Url: https://codereview.chromium.org/2867233002
Cr-Commit-Position: refs/heads/master@{#45238}