Add support for tracing in browser tests.

content/public/common/content_switches.cc

Index: content/public/common/content_switches.cc
diff --git a/content/public/common/content_switches.cc b/content/public/common/content_switches.cc
index 1747428ae9ed723e27df0c3ee1f4e105c2d79893..555628c0af1e5e72d1e32dea7e9ece4825aee59f 100644
--- a/content/public/common/content_switches.cc
+++ b/content/public/common/content_switches.cc
@@ -446,7 +446,13 @@ const char kEnableServiceWorkerSync[]       = "enable-service-worker-sync";
 const char kEnableTcpFastOpen[]             = "enable-tcp-fastopen";
 
 // Enabled threaded compositing for layout tests.
-const char kEnableThreadedCompositing[] = "enable-threaded-compositing";
+const char kEnableThreadedCompositing[]     = "enable-threaded-compositing";
+
+// Enable tracing during the execution of browser tests.
+const char kEnableTracing[]                 = "enable-tracing";
+
+// The filename to write the output of the test tracing to.
+const char kEnableTracingOutput[]           = "enable-tracing-output";
 
 // Enable screen capturing support for MediaStream API.
 const char kEnableUserMediaScreenCapturing[] =