stylo: Store location information for keyframe rules.

So far, we only store location info for the whole Keyframes block, not for each
of the keyframe rule. Without this info, the devtool can't present the rules
on the devtool panel properly.

In this patch, we collect the source location info while parsing keyframe selector.
The binding function, Servo_KeyframesRule_GetKeyframe, is also fixed (and renamed
to Servo_KeyframesRule_GetKeyframeAt to match the fix) to accept line and column
parameters from Gecko, so we can pass/set them with the source location from Servo.

ports/geckolib/glue.rs

@@ -1509,11 +1509,16 @@ pub extern "C" fn Servo_KeyframesRule_GetCount(rule: RawServoKeyframesRuleBorrow
 }
 
 #[no_mangle]
-pub extern "C" fn Servo_KeyframesRule_GetKeyframe(rule: RawServoKeyframesRuleBorrowed, index: u32)
-                                                  -> RawServoKeyframeStrong {
-    read_locked_arc(rule, |rule: &KeyframesRule| {
-        rule.keyframes[index as usize].clone().into_strong()
-    })
+pub extern "C" fn Servo_KeyframesRule_GetKeyframeAt(rule: RawServoKeyframesRuleBorrowed, index: u32,
+                                                    line: *mut u32, column: *mut u32) -> RawServoKeyframeStrong {
+    let global_style_data = &*GLOBAL_STYLE_DATA;
+    let guard = global_style_data.shared_lock.read();
+    let key = Locked::<KeyframesRule>::as_arc(&rule).read_with(&guard)
+                  .keyframes[index as usize].clone();
+    let location = key.read_with(&guard).source_location;
+    *unsafe { line.as_mut().unwrap() } = location.line as u32;
+    *unsafe { column.as_mut().unwrap() } = location.column as u32;
+    key.into_strong()
 }
 
 #[no_mangle]