Allow preload scanners to be disabled by Settings.

Allow preload scanners to be disabled by Settings.

In slow environments, the preload scanners may well be slowing us down
by launching too many requests too early. By making them possible to
disable, we can experiment with potential benefits to not having them.

This will be used in finch trials to study the potential benefit of
this and through command line forcing in labs.

BUG=None
R=jochen@chromium.org,Yoav Weiss,japhet@chromium.org,pmeenan

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=197660

[gavinp, 2015-06-22 19:47:06]

jochen: PTAL.

japhet, yoav, pmeenan: FYI.

My one question: how often will we have to worry about not having a settings
object? I worry that the field trial has a path around it, but I suppose it's a
minor case in plugins?

[gavinp, 2015-06-22 21:17:53]

An example dependent CL using the Finch trial is at:
https://codereview.chromium.org/1199043002/

[gavinp, 2015-06-23 14:52:33]

My latest upload adds a unit test.

jochen, ptal!

[jochen (gone - plz use gerrit), 2015-06-23 14:52:33]

lgtm

[gavinp, 2015-06-23 15:01:36]

The CQ bit was checked by gavinp@chromium.org

[commit-bot: I haz the power, 2015-06-23 15:02:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1196193005/20001

[commit-bot: I haz the power, 2015-06-23 16:29:31]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197660