Auto merge of #17447 - emilio:root-disconnected-subtree, r=heycam,Manishearth

style: Be more strict when setting the root font size.

Before this commit, we assumed that if the element had no parent element, it was
the root of the document, which is plain false, since we can arrive there from,
let's say, getComputedStyle on a detached node.

Bug: 1374062
Reviewed-By: heycam
MozReview-Commit-ID: 65DxdzXgd0J

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ports/geckolib/glue.rs

@@ -1546,7 +1546,7 @@ pub extern "C" fn Servo_ComputedValues_Inherit(
             StyleBuilder::for_inheritance(reference,
                                           &data.default_computed_values());
         if for_text {
-            StyleAdjuster::new(&mut style, /* is_root = */ false)
+            StyleAdjuster::new(&mut style)
                 .adjust_for_text();
         }
 
@@ -1608,8 +1608,8 @@ pub extern "C" fn Servo_StyleSet_Drop(data: RawServoStyleSetOwned) {
 pub extern "C" fn Servo_StyleSet_CompatModeChanged(raw_data: RawServoStyleSetBorrowed) {
     let mut data = PerDocumentStyleData::from_ffi(raw_data).borrow_mut();
     let quirks_mode = unsafe {
-        (*(*data.stylist.device().pres_context).mDocument
-                                               .raw::<nsIDocument>()).mCompatMode
+        (*data.stylist.device().pres_context().mDocument.raw::<nsIDocument>())
+            .mCompatMode
     };
 
     data.stylist.set_quirks_mode(quirks_mode.into());