Auto merge of #7807 - glennw:pid, r=jdm

Make it possible for iframes to create their own pipeline ID.

This doesn't change any functionality, but it's the first step towards removing SubpageId.

Adding this change now will allow us to gradually change over code referencing subpage id rather than in one massive PR.

Introduces a namespace for pipeline ID generation - there is a namespace for the constellation thread, and one per script thread.

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components/layout/layout_task.rs

@@ -277,7 +277,7 @@ impl LayoutTaskFactory for LayoutTask {
                                              time_profiler_chan,
                                              mem_profiler_chan.clone());
 
-                let reporter_name = format!("layout-reporter-{}", id.0);
+                let reporter_name = format!("layout-reporter-{}", id);
                 mem_profiler_chan.run_with_memory_reporting(|| {
                     layout.start();
                 }, reporter_name, layout_chan.0, Msg::CollectReports);