FrameTestHelpers: support Settings-parameterized tests.

This consolidates the mechanism for running unit tests with and
without root layer scrolling enabled.  The same mechanism can be
used to parameterize anything in WebSettings.

BUG=490942
R=skobes@chromium.org

Review-Url: https://codereview.chromium.org/2154493002
Cr-Commit-Position: refs/heads/master@{#405626}
4 files changed