Port 1a3aab51

Original Commit Message:

    When generating an Assembler, you currently have two choices: Either
    let the Assembler allocate a growable internal buffer, which is owned
    by the Assembler. Or provide an externally allocated buffer, which
    cannot grow.
    This CL changes this interface to allow providing any implementation of
    a buffer. The provided buffer can be a view to an externally owned
    buffer, which still can grow.
    This will be used to split WebAssembly compilation and code submission.
    The buffer needs to be able to grow, but cannot be owned by the
    Assembler because it has to survive until the code is submitted.

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

Change-Id: Id9383db813b13ea1d9eab485724aeb55b08cdfee
Reviewed-on: https://chromium-review.googlesource.com/c/1416310Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#58865}