[webkit-dev] Settings and Preferences in layout tests

[Simon Fraser]

We have a lot of tests that poke internal settings, via testRunner:

    testRunner.setFrameFlatteningEnabled(true);

or window.internals:

    internals.settings.setPageScaleFactor(0.5, 0, 0);

and some that poke preferences, like:

    testRunner.overridePreference("WebKitUsesPageCachePreferenceKey", 1);

First, direct calls on testRunner that just set preferences should be migrated to internals.settings or testRunner.overridePreference calls, I think (I don't know if either is preferred).

Secondly, I see no guarantee that these settings and preferences are reset to their default values before the next test, and we don't seem to have a consistent strategy about how to do this.

For internal settings, we have InternalSettings::Backup methods. However, there's no enforcement that when changing a setting in a test for the first time, developers also add code to reset it in InternalSettings.

I looked at testRunner.overridePreference(), and it doesn't appear to reset the value at the end of the test.

So I think we need clearer rules here. I suggest:

* testRunner methods that just set preferences should migrate to internals.settings or testRunner.overridePreference
* we should choose between internals.settings or testRunner.overridePreference if that makes sense.
* we should enforce a policy that patches adding a settings/prefs toggle should, if necessary, add code to reset between tests.

Simon