FIX Slight amend to CSS for CMS sizing

I've changed the CSS so it's now mimicking the core CSS for the CMS.

Author: dhensby

css/GoogleMapField.css

@@ -2,8 +2,10 @@
 	position: relative;
 }
 .googlemapfield-map {
-	width: 640px;
+	width: 100%;
 	height: 480px;
 	border: 1px solid #b3b3b3;
-	margin-top: 16px;
+	margin-top: 8px;
+	box-sizing: border-box;
+	max-width: 512px;
 }