Add the first very basic bits of a geofencing API.

Source/modules/geolocation/CircularRegion.idl

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+[
+    RuntimeEnabled=Geofencing,
+    GarbageCollected,
+    Constructor(Dictionary init)
+] interface CircularRegion : GeofencingRegion {
+    const double MIN_RADIUS = 1.0;
+    const double MAX_RADIUS = 100.0;

[jochen (gone - plz use gerrit), 2014/07/10 11:38:51]

is it clear for geo ppl what unit the radius is in?

[Marijn Kruisselbrink, 2014/07/10 12:36:00]

I don't know. Are IDL files supposed to document the API? The spec will of
course make it clear what units these are in. Also I'm not entirely convinced
yet it makes sense to have these constants in the first place, but that's a
separate issue.
(for now the only bits of "spec" there are are at 
https://github.com/mkruisselbrink/Geofencing/blob/master/explainer.md and
https://gist.github.com/mkruisselbrink/c5c5cd80f27f3158f451)

[Peter Beverloo, 2014/07/10 13:17:27]

FWIW, the IDL files define the Web Exposed API surface of this feature. Would a
JavaScript developer reading the following code understand what the radius will
be?

var my_region = new CircularRegion({
    id: "work",
    latitude: 51.4947691,
    longitude: -0.1463945,
    radius: CircularRegion.MAX_RADIUS
});

+
+    readonly attribute double latitude;
+    readonly attribute double longitude;
+    readonly attribute double radius;
+};