Port 1a1f4e1e

Original Commit Message:

    Refactor the AllocateAssemblerBuffer helper for the new Assembler API.
    This is the only non-mechanical part, all other callsites that create
    Assembler instances can be trivially changed to the new API. This will
    be done in a separate CL.

R=clemensh@chromium.org, joransiu@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Change-Id: I8e22f8c2b6c2b1b9158969d28d4edf291a84bcf0
Reviewed-on: https://chromium-review.googlesource.com/c/1416952
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#58897}