Lift the declaration of the heap allocation request list and the method
which adds to the list up to AssemblerBase.

Change-Id: I099260425af8cb579144998c71c538f19ba00e65
Reviewed-on: https://chromium-review.googlesource.com/1098959Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53708}