Auto merge of #15020 - jdm:external_script_line, r=Ms2ger

Do not use the script element's line number for external scripts.

This was yielding incorrect line numbers when looking at JS backtraces in gdb.

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] There are tests for these changes

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tests/wpt/mozilla/tests/mozilla/resources/external.js

@@ -0,0 +1,4 @@
+
+
+
+this_is_a_js_error

tests/wpt/mozilla/tests/mozilla/track_line.html

@@ -6,12 +6,18 @@
 <script>
 setup({allow_uncaught_exception:true});
 var t = async_test("error event has proper line number");
+var errors = 0;
+var expected_lines = [21, 4];
 window.addEventListener('error', t.step_func(function(e) {
     assert_true(e instanceof ErrorEvent);
-    assert_equals(e.lineno, 16);
-    t.done();
+    assert_equals(e.lineno, expected_lines[errors]);
+    errors++;
+    if (errors == 2) {
+        t.done();
+    }
 }), true);
 </script>
 <script>
 this_is_a_js_error
 </script>
+<script src="resources/external.js"></script>