Drop the required base::MessageLoopForIO when creating a BaseMessageLoop.

This patch adds a new default constructor to brillo::BaseMessageLoop
dropping the requirement to create and pass a base::MessageLoopForIO.
Instead, the default constructor will create and keep its own copy.

Bug: 26987669
TEST=`m` on edison-eng

Change-Id: Ibd8a1d98cbbc1cc1d39f5e90412902edd6840a45
4 files changed