Add test runner support for overriding web_security_enabled preference.

Have TestRunner::OverridePreference() recognize "WebKitWebSecurityEnabled"
as a boolean preference which controls the web_security_enabled web
preference.

R=mkwst,jochen
BUG=254993

Review URL: https://codereview.chromium.org/601513002

Cr-Commit-Position: refs/heads/master@{#296543}
4 files changed