libchromeos: add support for synchronous PropertySet::Get to dbus

This CL adds the support for the newly added GetAndBlock function
in libchrome dbus library.
This will allow us to use a synchronous version of PropertySet::Get.

Bug: 24131409
TEST=run tests on a veyron_jaq chromebook

Change-Id: I926e480c14ff2627fdad5351e438e86a03a721aa
1 file changed