Part 3: Merged FakedDBusThreadManager with DBusThreadManager.

BUG=401192
TEST=existing tests
TBR=sky,stevenjb,keybuk

Review URL: https://codereview.chromium.org/477663004

Cr-Commit-Position: refs/heads/master@{#292072}
74 files changed