Making the ChildConnectionAllocator clean connection itself.

The ChildConnectionAllocator relies on the ChildProcessLauncherHelper
for clearing up connections when they get disconnected.
Moving that logic to the ChildConnectionAllocator.
For that I had to change the deathcallback so it is always invoked
(even if the process was stopped by the client).

Bug: 702316
Change-Id: I9c0a229471ef3c80359e60da628bff32f51a27f2
Reviewed-on: https://chromium-review.googlesource.com/544796
Reviewed-by: Bo Liu <boliu@chromium.org>
Commit-Queue: Jay Civelli <jcivelli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#482026}
3 files changed