[content shell] add --no-timeout option

content/shell/shell_switches.cc

Index: content/shell/shell_switches.cc
diff --git a/content/shell/shell_switches.cc b/content/shell/shell_switches.cc
index e3cffb3968f60a5c15652ac239dbb592f6aac940..07daec5b2f76d78f0a83a3624702d70e881d432b 100644
--- a/content/shell/shell_switches.cc
+++ b/content/shell/shell_switches.cc
@@ -15,4 +15,7 @@ const char kContentBrowserTest[] = "browser-test";
 // Request pages to be dumped as text once they finished loading.
 const char kDumpRenderTree[] = "dump-render-tree";
 
+// Disables the timeout for layout tests.
+const char kNoTimeout[] = "no-timeout";
+
 }  // namespace switches