This CL introduces a new MemoryController that will be used to control
the size of external memory (array buffers and external string for now).

Bug: chromium:845409
Change-Id: I119506ce0243ac33cec2b783b888b53ee11225a9
Reviewed-on: https://chromium-review.googlesource.com/1156393
Commit-Queue: Rodrigo Bruno <rfbpb@google.com>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54854}