Opt-in stripping of line numbers from console messages of layout tests.

components/test_runner/layout_test_runtime_flags.h

Index: components/test_runner/layout_test_runtime_flags.h
diff --git a/components/test_runner/layout_test_runtime_flags.h b/components/test_runner/layout_test_runtime_flags.h
index 995148c4ade8fd02672d794185b874faa1ba24e4..180b2b8575644c472a97486220ec1afed6f6dbe0 100644
--- a/components/test_runner/layout_test_runtime_flags.h
+++ b/components/test_runner/layout_test_runtime_flags.h
@@ -155,6 +155,10 @@ class TEST_RUNNER_EXPORT LayoutTestRuntimeFlags {
   // be part of test output.
   DEFINE_BOOL_LAYOUT_TEST_RUNTIME_FLAG(dump_console_messages)
 
+  // If true, line numbers will be included in the console messages produced
+  // by the test page.
+  DEFINE_BOOL_LAYOUT_TEST_RUNTIME_FLAG(include_line_numbers_in_console_messages)
+
   // Desired return value of WebFrameClient::runModalBeforeUnloadDialog.
   DEFINE_BOOL_LAYOUT_TEST_RUNTIME_FLAG(
       stay_on_page_after_handling_before_unload)