Allow mojo proxy objects to be created unbound for testing.

There are 5 tests in the internal repo (one per class) that subclass
these objects for mocks. The new PendingAssociatedRemote in the
constructor takes a lot of boilerplate to initialize and since the
constructor asserts on is_valid() it can't take unbound remote. That
greatly complicates the test setup even though the tests don't
actually use the remote.

R=proberge

Bug: 1034077
Change-Id: Ica228fe42dddd09d75613a58c831d8c8f1ab504f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1967440
Reviewed-by: proberge <proberge@chromium.org>
Commit-Queue: Joe Mason <joenotcharles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#724884}
10 files changed