[ChromeDriver] Select the main frame if a non-existant child frame is targeted.

BUG=169092
TEST=run_py_tests.py


Review URL: https://chromiumcodereview.appspot.com/12226026

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@181233 0039d316-1c4b-4281-b951-d872f2087c98
12 files changed