Add ExecuteJavascript() method to PyUITestBase

This allows us to evaluate JavaScript expressions in the renderer and
read values out of the DOM of the page, which is useful for testing things
like the PasswordManager.

    R=nirimesh@chromium.org, jrg@chromium.org, alyssad@chromium.org
    TEST=chrome/test/functional/test_execute_javascript.py
    BUG=none

Review URL: http://codereview.chromium.org/3012039

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@55013 0039d316-1c4b-4281-b951-d872f2087c98