[heap] Add a CodePageCollectionMemoryModificationScope to collect unprotected code pages on the fly.

Bug: chromium:774108
Change-Id: I95bfe672df13a6b8f40ba258c9ea42c6b18f6138
Reviewed-on: https://chromium-review.googlesource.com/949482Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51765}