Replace gtest_prod.h include with our own FRIEND_TEST macro.
Replace gtest_prod.h include with our own FRIEND_TEST macro.

This small bit of duplication avoids depending on any part of GTest in
production code.

TBR=phoglund
BUG=1395

Review URL: https://webrtc-codereview.appspot.com/1098013

git-svn-id: 03ae4fbe531b1eefc9d815f31e49022782c42458@3541 4adac7df-926f-26a2-2b94-8c16560cd09d

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