Bug 1616364 - initailize thread profiler on Compositor thread only on…

…ce r=gw

[import_pr] From servo#3864

Differential Revision: https://phabricator.services.mozilla.com/D63242

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/e106dc52d496cbcadece6b6dcab57071ac815644

webrender/src/renderer.rs

@@ -1969,6 +1969,10 @@ impl Renderer {
         shaders: Option<&mut WrShaders>,
         start_size: DeviceIntSize,
     ) -> Result<(Self, RenderApiSender), RendererError> {
+        if !wr_has_been_initialized() {
+            register_thread_with_profiler("Compositor".to_owned());
+        }
+
         HAS_BEEN_INITIALIZED.store(true, Ordering::SeqCst);
 
         let (api_tx, api_rx) = channel::msg_channel()?;
@@ -2028,8 +2032,6 @@ impl Renderer {
         let max_texture_size = device.max_texture_size();
         let max_texture_layers = device.max_texture_layers();
 
-        register_thread_with_profiler("Compositor".to_owned());
-
         device.begin_frame();
 
         let shaders = match shaders {