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>().

Previous version: https://chromium-review.googlesource.com/1219025

BUG=v8:7308
TBR=ishell@chromium.org, ulan@chromium.org, ahaas@chromium.org, yangguo@chromium.org, tebbi@chromium.org

Change-Id: I503d4a2a3a68f85e9e02e1c2f9fc1c4187c8e9a1
Reviewed-on: https://chromium-review.googlesource.com/1226800Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55934}