Bug 1651705: Part 2 - Create NativeWeakPtr to safely access native ob…

…jects owned by Java objects across multiple threads; r=geckoview-reviewers,agi

* We rename the existing `NativePtr` struct to `NativePtrTraits`, as that is
  more descriptive of what that code actually does;
* We introduce `NativeWeakPtr` as a smart pointer type that holds a pointer
  to an object and allows its access in a thread-safe way. See comments.
* We replace some explicit uses of template types with type deduction via
  `auto` and `decltype(auto)`. This allows for more use of forward declarations.

Differential Revision: https://phabricator.services.mozilla.com/D87361