Currently we initialize the allocated buffer to be full of 0s, which
adds significant overhead.

TypedArrayConstructByArrayLike will always either fully initialize the
buffer, or throw an exception, in which case the buffer will not be
leaked to user code.

The length of the new TypedArray (and thus the buffer) is derived from
the length of the source Array/TypedArray, so we know that we will
always set every byte of the new buffer, or throw trying.

Bug:v8:5977

Change-Id: I8ceaa883cfad85f8708a5bdaada3ce463d97e007
Reviewed-on: https://chromium-review.googlesource.com/469348Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44447}