This reverts commit ad72d195.

Reason for revert: Build failures on *san

Original change's description:
> [in-place weak refs] Fix MaybeObject function names
> 
> 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/1219025
> Reviewed-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}

TBR=ulan@chromium.org,marja@chromium.org,yangguo@chromium.org,ahaas@chromium.org,tebbi@chromium.org,ishell@chromium.org

Change-Id: I054b578518e3f6fd7dbcddf0b56cc018726c1e7a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:7308
Reviewed-on: https://chromium-review.googlesource.com/1226874Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55918}