Fix net_unittests when run in a virtualenv.

net/test/python_utils.cc

Index: net/test/python_utils.cc
diff --git a/net/test/python_utils.cc b/net/test/python_utils.cc
index 868abbb4e35de62e08eeb05529d32a960766269d..57429626b17c2bd30633a1f054e10e3a4476cb6e 100644
--- a/net/test/python_utils.cc
+++ b/net/test/python_utils.cc
@@ -23,6 +23,7 @@
 #endif
 
 const char kPythonPathEnv[] = "PYTHONPATH";
+const char kPythonVirtualEnv[] = "VIRTUAL_ENV";
 
 void ClearPythonPath() {
   std::unique_ptr<base::Environment> env(base::Environment::Create());
@@ -102,6 +103,10 @@ struct PythonExePath {
 static base::LazyInstance<PythonExePath>::Leaky g_python_path;
 #endif
 
+bool IsInPythonVirtualEnv() {
+  return base::Environment::Create()->HasVar(kPythonVirtualEnv);
+}
+
 bool GetPythonCommand(base::CommandLine* python_cmd) {
   DCHECK(python_cmd);
 
@@ -121,8 +126,10 @@ bool GetPythonCommand(base::CommandLine* python_cmd) {
   // gtest output in buildbot log files. See http://crbug.com/147368.
   python_cmd->AppendArg("-u");
 
-  // Prevent using system-installed libraries. Use hermetic versioned copies.
-  python_cmd->AppendArg("-S");
+  if (!IsInPythonVirtualEnv()) {
+    // Prevent using system-installed libraries. Use hermetic versioned copies.
+    python_cmd->AppendArg("-S");
+  }
 
   return true;
 }