[Startup Tracing] Add --trace-config-file flag

components/tracing/tracing_switches.cc

Index: components/tracing/tracing_switches.cc
diff --git a/components/tracing/tracing_switches.cc b/components/tracing/tracing_switches.cc
index fcec5eeed737598e47c0ac831c45edd0fda39312..ba3c1b158fb1a9ffe12308138d6bd25b18bcb460 100644
--- a/components/tracing/tracing_switches.cc
+++ b/components/tracing/tracing_switches.cc
@@ -6,6 +6,10 @@
 
 namespace switches {
 
+// Causes TRACE_EVENT flags to be recorded from startup.
+// This flag will be ignored if --trace-startup or --trace-shutdown is provided.
+const char kTraceConfigFile[]               = "trace-config-file";
+
 // Causes TRACE_EVENT flags to be recorded beginning with shutdown. Optionally,
 // can specify the specific trace categories to include (e.g.
 // --trace-shutdown=base,net) otherwise, all events are recorded.