Fixes a crash that happens when calling postMessage on an empty typed
array.

GetBuffer should only call MaterializeArrayBuffer for on-heap buffers,
but the on-heap check is slightly wrong. This CL moves the on-heap check
logic to the JSTypedArray class so that other parts of the codebase
don't need to worry about how that is determined.

Also add some dchecks to materialize itself. It should only receive
on-heap buffers and should always transform them to off-heap buffers.
There is also no reason for it to be static, so change that here too.

Bug: chromium:797588
Change-Id: Icd88a5b68e424d82c9f1f7889ca42a40a72a1bdc
Reviewed-on: https://chromium-review.googlesource.com/995898
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52388}