E.g., "ToWeakHeapObject" was misleading, since it didn't convert to a weak heap
object, instead returned a weakly pointed heap object. Change the function names
(in this case, to "GetHeapObjectIfWeak") to reflect this.

Also make casts explicit, if a MaybeObject is an Object, we can call cast<Object>().

BUG=v8:7308

Change-Id: I4ef078572b4f4415afe7e2e706d3bd684e16e47d
Reviewed-on: https://chromium-review.googlesource.com/1219025Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55906}