Add external/googletest.

This is going to be replacing external/gtest and external/gmock.

Test: None (there aren't any build files yet)
Bug: http://b/16574165
diff --git a/default.xml b/default.xml
index 154b12c..771f97e 100644
--- a/default.xml
+++ b/default.xml
@@ -71,6 +71,7 @@
   <project path="external/gmock" name="platform/external/gmock" groups="bdk" />
   <project path="external/google-benchmark" name="platform/external/google-benchmark" groups="bdk" />
   <project path="external/google-breakpad" name="platform/external/google-breakpad" groups="dragon,bdk" />
+  <project path="external/googletest" name="platform/external/googletest" groups="dragon,bdk" />
   <project path="external/gtest" name="platform/external/gtest" groups="bdk" />
   <project path="external/icu" name="platform/external/icu" groups="bdk" />
   <project path="external/iproute2" name="platform/external/iproute2" groups="bdk" />