Fixed comparison between signed and unsigned.
Added back GTEST_USE_OWN_TR1_TUPLE for libprocess.

Review: https://reviews.apache.org/r/18866
Added back GTEST_USE_OWN_TR1_TUPLE for Mesos.

Review: https://reviews.apache.org/r/18867
Fixed SystemCpuUsage isolator test.

Review: https://reviews.apache.org/r/18885