Update the googletest version

We now use googletest as a git submodule instead
of the old way of converting it into a single header
and including it in the source.

This removes the dependence on googleflags for tests
to be enabled.

Change-Id: If25ea3ba7a39c2b8ba9c6effbed3e7173361b6b1

third_party/README.md

@@ -0,0 +1 @@
+This directory is used for keeping track of our third party dependencies as git submodules.