Revert of  Clean up public interface of AttachmentBrokerUnprivileged. (patchset #6 id:120001 of https://codereview.chromium.org/1679763002/ )

Reason for revert:
failures on Chromium Memory FYI:

http://build.chromium.org/p/chromium.memory.fyi/
Failure notification for "memory test: remoting" on "Chromium Mac (valgrind)(2)".
Please see if the failures are related to your commit and take appropriate actions (e.g. revert, update suppressions, notify sheriff, etc.).

For more info on the memory waterfall please see these links:
http://dev.chromium.org/developers/tree-sheriffs/sheriff-details-chromium/memory-sheriff
http://dev.chromium.org/developers/how-tos/using-valgrind
http://dev.chromium.org/developers/how-tos/using-valgrind/threadsanitizer

By the way, the current memory sheriff is on the CC list.

http://build.chromium.org/p/chromium.memory.fyi/builders/Chromium%20Mac%20%28valgrind%29%282%29/builds/37788

Original issue's description:
> Clean up public interface of AttachmentBrokerUnprivileged.
>
> In the old interface, a static factory method returns a scoped_ptr, and the
> caller had to manage the lifetime. Since this is a global object with minimal
> memory footprint, and is required to outlive every IPC::Channel, it's much
> easier for the global to never be destroyed. This also matches the interface for
> AttachmentBrokerPrivileged.
>
> BUG=584297
>
> Committed: https://crrev.com/11fea2242b3a197993dbd5a1f977f9a31c6b98e4
> Cr-Commit-Position: refs/heads/master@{#375674}

TBR=tsepez@chromium.org,avi@chromium.org,mseaborn@chromium.org,sergeyu@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=584297

Review URL: https://codereview.chromium.org/1704743002

Cr-Commit-Position: refs/heads/master@{#375739}
17 files changed