This adds a convenience overload for `EscapableHandleScope::Escape()`
which moves `MaybeLocal<T>`s into the outer scope, like a regular
`Local<T>`.

This basically moves the syntactic clutter of having to write
`maybe_local.FromMaybe(Local<Foo>())` instead of just `maybe_local`
to a central location.

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I1d87d75c6564b10e8ec34957bdd3eac46ffea917
Reviewed-on: https://chromium-review.googlesource.com/1056529Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53415}