servo: Merge #8971 - Add pipeline information to CSS error reporting (from jdm:expose-css-errors-1); r=jdm

Rebase of #8838.

Source-Repo: https://github.com/servo/servo
Source-Revision: d11f96e27074b0130760a02d39d2da4e003c820e

servo/components/script_traits/lib.rs

@@ -151,6 +151,8 @@ pub enum ConstellationControlMsg {
         /// The pipeline that contains a frame loading the target pipeline.
         parent: PipelineId
     },
+    /// Report an error from a CSS parser for the given pipeline
+    ReportCSSError(PipelineId, String, usize, usize, String),
 }
 
 /// The type of input represented by a multi-touch event.