Mojo: Make a public test_support shared library.

The test runner can then inject dependencies into it (just as we do for
system and gles2).

This removes public/tests's base dependency.

R=sky@chromium.org

Review URL: https://codereview.chromium.org/160003007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251115 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251115