Auto merge of #10399 - jdm:pb, r=jdm

Private browsing - Initial steps

Rebase of #10160.

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components/compositing/pipeline.rs

@@ -58,6 +58,7 @@ pub struct Pipeline {
     /// animations cause composites to be continually scheduled.
     pub running_animations: bool,
     pub children: Vec<FrameId>,
+    pub is_private: bool,
 }
 
 /// The subset of the pipeline that is needed for layer composition.
@@ -275,6 +276,7 @@ impl Pipeline {
             children: vec!(),
             size: size,
             running_animations: false,
+            is_private: false,
         }
     }