Fix webkitpy unittests.

Fix webkitpy unittests.

-Fix all the things that change now that content shell is the default driver.
-Have the default value for --ignore-flaky-tests be None instead of 'default'.
Having it be 'default' doesn't gain anything and would require updating a bunch
of unittests to set the ignore_flaky_tests to 'default' in their options.

TBR=dpranke@chromium.org,jochen@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=151014

[ojan, 2013-05-23 20:41:08]

Committed patchset #1 manually as r151014 (presubmit successful).

[Dirk Pranke, 2013-05-23 23:36:15]

lgtm.