Support nullable types in mojom.
This CL updates the lexer, parser and also the internal representation of mojom modules. The changes to the generators will be in a separate CL.
This CL doesn't change data_tests.py because it has been broken for quite a while. We could fix it in a separate CL if we think that it is still useful.
BUG=324170
TEST=those updated test files.
Review URL: https://codereview.chromium.org/
437643002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287997 0039d316-1c4b-4281-b951-d872f2087c98