This is the first step towards full huge typed array support in V8.
Before this change, the JSTypedArray::length and the elements backing
store length (FixedTypedArrayBase::length) were used more or less
interchangeably to determine the number of elements in a JSTypedArray.

With this change we disentangle these two lengths, and instead make
JSTypedArray::length authoritative. For on-heap typed arrays, the
FixedTypedArrayBase::length will remain the number of elements in the
backing store, but for the off-heap typed arrays, this length will be
set to 0 (matching the fact that the FixedTypedArrayBase instance does
not contain any elements itself).

This also unifies the JSTypedArray::set_/length() and length_value()
methods to only have JSTypedArray::set_/length() which returns/takes
size_t values. Currently this still requires the values to be in Smi
range, but later we will extend this to allow arbitrary size_t values
(in the safe integer range).

Bug: v8:4153, v8:7881
Change-Id: Iff9089130bb31fa9e08e0cf913e7ab52c3dbf107
Cq-Include-Trybots: luci.chromium.try:linux-blink-rel
Doc: http://doc/1Z-wM2qwvAuxH46e9ivtkYvKzzwYZg8ymm0x0wJaomow
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1543729
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60648}