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This allows the corresponding factory to return refptrs instead of raw
pointers, and lets remove the explicit Destroy() method.
One downside is that now all implementations have to be refcounted, but
this only affects some test implementations.

One particular test implementation, TestHttpBridge[Factory], was unused
and is deleted.

Bug: 951350
Change-Id: I8c581136389e2ce470786dffefa3145e88c5d19c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2466237
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Victor Vianna <victorvianna@google.com>
Cr-Commit-Position: refs/heads/master@{#816494}