compositor: Make PipelineDetails and pending paint metrics per-WebView (#35701)

This is one of the first big steps toward making the compositor work
per-WebView. It moves the collection of pipelines into the per-WebView
data structure in the compositor as well as the pending paint metrics.

This means that more messages need to carry information about the
WebView they apply to. Note that there are still a few places that we
need to map from `PipelineId` to `WebViewId`, so this also includes a
shared mapping which tracks this. The mapping can be removed once event
handling is fully per-WebView.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Delan Azabani <dazabani@igalia.com>

components/script/script_thread.rs

@@ -3051,6 +3051,7 @@ impl ScriptThread {
         };
 
         let paint_time_metrics = PaintTimeMetrics::new(
+            incomplete.top_level_browsing_context_id,
             incomplete.pipeline_id,
             self.senders.time_profiler_sender.clone(),
             self.senders.layout_to_constellation_ipc_sender.clone(),