[Mac][Backport] Partial backport of upstream to make storage_monitor a component.

This backport the mac specific bits needed to build on Mac.

URL : https://codereview.chromium.org/152343005/
Author : Thiago Santos
Merge pull request #132 from darktears/mac-build-fix

[Mac][Backport] Partial backport of upstream to make storage_monitor a c...