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Previously ScopedServiceBinding was using fidl::Binding instead of
BindingSet, which didn't allow multiple bindings. Also added some new
unittests for ServiceDirectory to validate cases that were not handled
previously. That required an update in ComponentContext class: it now
can handle the case when the target directory is gone.

Change-Id: I7975275bebfba716d97c3f41d13046172385aff7
Reviewed-on: https://chromium-review.googlesource.com/1125308Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572426}