This CL is purely refactoring, no behavior changes.

Remove InitializeBasedOnLength and combine it with a new Stub-ified
TypedArrayInitialize which now allocates the buffer in both the
on-heap and off-heap cases.

Add TypedArrayInitializeWithBuffer because this was essentially a
special case that didn't share much logic with Initialize.
Factor out the common pieces into SetupTypedArray and AttachBuffer.

We can also always pass in the elementsSize, so there is no need
to calculate this again. LoadMapAndElementsSize is changed to 
LoadMapForType.

This reduces code size by ~8k.

Bug: chromium:711275,chromium:701768
Change-Id: I6ad8701e9c72f53bfd9484725fb82055be568c25
Reviewed-on: https://chromium-review.googlesource.com/483481
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44850}