[chromedriver] Add shadow DOM support to chromedriver.

BUG=chromedriver:852

TEST= added some unit tests (beginning with testShadowDom) to chrome/test/chromedriver/test/run_py_tests.py

R=samuong@chromium.org

Review URL: https://codereview.chromium.org/480483002

Cr-Commit-Position: refs/heads/master@{#309002}