Attempt 2 at : Makes tests either use mock compositor or mock

chrome/common/chrome_switches.cc

Index: chrome/common/chrome_switches.cc
diff --git a/chrome/common/chrome_switches.cc b/chrome/common/chrome_switches.cc
index 1b488e1d179a611d5777041e9f223d302574aa55..cf14a5054b3ae16056feb00af56ed76c1710932c 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -1286,6 +1286,11 @@ const char kEnablePrintPreview[]            = "enable-print-preview";
 // Enables the benchmarking extensions.
 const char kEnableBenchmarking[]            = "enable-benchmarking";
 
+#if defined(USE_AURA)
+// Forces usage of the test compositor. Needed to run ui tests on bots.
+extern const char kTestCompositor[]         = "test-compositor";
+#endif
+
 bool IsPrintPreviewEnabled() {
   if (CommandLine::ForCurrentProcess()->HasSwitch(kDisablePrintPreview))
     return false;