Like Heap::InFromSpace, it doesn't use any members, so should be made
static. Also clean up call sites to not call via a heap pointer.

Change-Id: If55484ddac51351d789c73093f1f7ebf1c568bea
Reviewed-on: https://chromium-review.googlesource.com/1088618Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53842}