GitLab

This CL adds DeferredDestroyStrongBindingSet such that mojom::Foo impl
will only be destroyed when:
- mojom::Foo connection error happens, AND FooImpl calls the
  DestroyCallback, or
- DeferredDestroyStrongBindingSet is destroyed.

This helps solve tricky lifetime issues where we do not want destroy the
implementation immediately.

For example, currently when connection error on mojom::CdmFactory
happens, CdmFactoryImpl will be destroyed, which will also destroy all
CDMs owned by CdmFactoryImpl. However, this could happen before MojoCdm
is stopped in the render process, causing unexpected behavior. See the
bug for more details.

This CL also fixes the CdmFactoryImpl lifetime issue by using the
DeferredDestroyStrongBindingSet, and let CdmFactoryImpl only call the
DestroyCallback when no outstanding CDMs exist.

For historical context, mojo::StrongBindingSet was originally added
also to solve lifetime issues around media::mojom::InterfaceFactory, the
predecessor of media::mojom::CdmFactory.
See https://codereview.chromium.org/2530613003/.

Bug: 821171
Test: Added new BindingSetTest cases.
Change-Id: I24a660e8b62720f7fd772db1bafb9499cb71428c
Reviewed-on: https://chromium-review.googlesource.com/969655
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548704}