Updating gtest from 1.6.0 to 1.7.0.

This is required in order to build libvpx on OS X Mavericks where gcc
compiler is deleted, clang (3.3) is the default now.

Using unmodified source files from gtest-1.7.0/fused-src folder.

Change-Id: I3d5f7278149c904e48737327daf7097a8bb0b390