Integrate gumbo_parser with gtest.

This adds the support to build with gtest submodule through gyp+ninja.

To generate the new ninja files, run:
$ gyp --depth=. gumbo_parser.gyp

To build use the following command line:
$ ninja -C out/Default gumbo_parser_unittests

To run use this command line:
$ out/Default/gump_parser_unittests

Signed-off-by: Thiago Farina <tfarina@chromium.org>