[chromedriver] Re-enable window & screenshot tests on HEAD.

Note: Not re-enabling these tests on M57 because the revision build used for v57tests on bot is old and does not contain the new fixes.

BUG=chromedriver:1625

Review-Url: https://codereview.chromium.org/2726903002
Cr-Commit-Position: refs/heads/master@{#454020}
2 files changed