Move tracing.mojom to public SDK.

R=tonyg@chromium.org

Review URL: https://codereview.chromium.org/1212813003.
diff --git a/shell/BUILD.gn b/shell/BUILD.gn
index add81bd..e6aa87d 100644
--- a/shell/BUILD.gn
+++ b/shell/BUILD.gn
@@ -160,9 +160,9 @@
     "//mojo/public/cpp/bindings",
     "//mojo/public/interfaces/application",
     "//mojo/services/network/public/interfaces",
-    "//shell/application_manager",
+    "//mojo/services/tracing/public/interfaces",
     "//services/url_response_disk_cache",
-    "//services/tracing:bindings",
+    "//shell/application_manager",
     "//url",
   ]