Auto merge of #10486 - notriddle:overflow_premature_clip, r=pcwalton

Take transform:translate into account when computing clipping regions.

Note that this only works for translation; a more general fix would
require major changes to how display lists work.

Closes #10431?

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components/gfx/display_list/mod.rs

@@ -1388,7 +1388,7 @@ impl DisplayItem {
 
 impl fmt::Debug for DisplayItem {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "{} @ {:?}",
+        write!(f, "{} @ {:?} {:?}",
             match *self {
                 DisplayItem::SolidColorClass(ref solid_color) =>
                     format!("SolidColor rgba({}, {}, {}, {})",
@@ -1408,6 +1408,7 @@ impl fmt::Debug for DisplayItem {
                 DisplayItem::IframeClass(_) => "Iframe".to_owned(),
             },
             self.bounds(),
+            self.base().clip
         )
     }
 }