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When renderer process is created, BackgroundTracingManagerImpl creates
closure for constructing of BackgroundTracingAgentClientImpl and creates
its instance lazily only when any agent observers are registered.
But it never releases if none of agent observer exists so it remained
as leaked memory.

To release pending constructor, BackgroundTracingAgentClientImpl
registers a disconnect handler to BackgroundTracingAgentProvider
then it releases when it is disconnected.
BackgroundTracingAgentProvider would be disconnected when the process
is gone.

Change-Id: I880c31caa7731dd3429c089ff523da5f846af00c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2065195
Commit-Queue: Darin Fisher <darin@chromium.org>
Reviewed-by: Darin Fisher <darin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745142}