Summary:
Adds gtest fused source code into `third-party` directory. No manual changes.

gtest latest released 1.7 has clang dev compilation errors. Trunk version requires only one disabled warning (-Wno-missing-field-initializers)

Fused code is made as described here https://fburl.com/90806322
Details about why we need gtest source code instead of precompiled library https://fburl.com/90805763
Source used from http://googletest.googlecode.com/svn/trunk

Test Plan:
Build and notice no errors. Also check in logs that gtest-all.o being compiled gtest-all.o.
```lang=bash
% USE_CLANG=1 make all
```

Reviewers: lgalanis, yufei.zhu, rven, sdong, igor, meyering

Reviewed By: meyering

Subscribers: meyering, yhchiang, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D33345