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..d02c3e58e9259d8d7cf2f8625e120544c9b6d749 100644
--- a/content/public/common/content_switches.cc
+++ b/content/public/common/content_switches.cc
@@ -448,6 +448,12 @@ const char kEnableTcpFastOpen[]             = "enable-tcp-fastopen";
 // Enabled threaded compositing for layout tests.
 const char kEnableThreadedCompositing[] = "enable-threaded-compositing";
 
+// Enable tracing during the execution of browser tests.
+const char kEnableTracing[] = "enable-tracing";

[jam, 2014/08/27 03:33:59]

nit: here and below, almost all flags have the '=' at the same column, please
follow that

+
+// 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[] =
     "enable-usermedia-screen-capturing";