Auto merge of #14173 - asajeffrey:script-thread-stores-top-level-frame-id, r=paulrouget

Report panics using the top-level frame id rather than the pipeline id

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At the moment, we report panics from script and layout using the root pipeline id. Once we are sharing more script threads, there won't be a root pipeline id any more. The plan is only to share script threads in the same tab, so there will be a top-level frame id that all the pipelines have in common. This PR reports panics using that top-level frame id.

This mostly makes a difference to the browser API: rather than targeting a `mozbrowsererror` event at the root iframe of the script thread that panicked, it targets it at the containing mozbrowser iframe.

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because we don't test crash reporting

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components/layout_thread/lib.rs

@@ -42,7 +42,6 @@ extern crate selectors;
 extern crate serde_json;
 extern crate servo_url;
 extern crate style;
-extern crate style_traits;
 extern crate util;
 extern crate webrender_traits;
 
@@ -80,7 +79,7 @@ use layout::webrender_helpers::{WebRenderDisplayListConverter, WebRenderFrameBui
 use layout::wrapper::LayoutNodeLayoutData;
 use layout::wrapper::drop_style_and_layout_data;
 use layout_traits::LayoutThreadFactory;
-use msg::constellation_msg::PipelineId;
+use msg::constellation_msg::{FrameId, PipelineId};
 use net_traits::image_cache_thread::{ImageCacheChan, ImageCacheResult, ImageCacheThread};
 use net_traits::image_cache_thread::UsePlaceholder;
 use parking_lot::RwLock;
@@ -235,6 +234,7 @@ impl LayoutThreadFactory for LayoutThread {
 
     /// Spawns a new layout thread.
     fn create(id: PipelineId,
+              top_level_frame_id: Option<FrameId>,
               url: ServoUrl,
               is_iframe: bool,
               chan: (Sender<Msg>, Receiver<Msg>),
@@ -251,7 +251,11 @@ impl LayoutThreadFactory for LayoutThread {
         thread::spawn_named(format!("LayoutThread {:?}", id),
                       move || {
             thread_state::initialize(thread_state::LAYOUT);
-            PipelineId::install(id);
+
+            if let Some(top_level_frame_id) = top_level_frame_id {
+                FrameId::install(top_level_frame_id);
+            }
+
             { // Ensures layout thread is destroyed before we send shutdown message
                 let sender = chan.0;
                 let layout = LayoutThread::new(id,
@@ -718,6 +722,7 @@ impl LayoutThread {
 
     fn create_layout_thread(&self, info: NewLayoutThreadInfo) {
         LayoutThread::create(info.id,
+                             FrameId::installed(),
                              info.url.clone(),
                              info.is_parent,
                              info.layout_pair,