[lacros] Make frame mojo::Remote objects for passing frames stable.

There is a flaky crash in the destructor of
VideoFrameHandlerProxyLacros::AccessPermissionProxyMap where client_ is
not set. It appears to be due to the std::map<> containing mojo remotes
directly. By changing it to contain std::unique_ptr<>s of the mojo
remotes, we ensure that the mojo::Remote objects are stable in memory
and that no uninitialized remotes or std::move() happens on the actual
mojo objects.

This CL also removes an accidental inherritance on
ScopedAccessPermissionMap which may also have caused the crashes.

Bug: chromium:1223367
Change-Id: I14198afc24864e29ad74af9c504237df8fc3f23f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3009471
Reviewed-by: Wei Lee <wtlee@chromium.org>
Commit-Queue: Wei Lee <wtlee@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#899093}