Only copy the plugin if it doesn't already exist.

chrome/test/ui/npapi_test_helper.cc

Index: chrome/test/ui/npapi_test_helper.cc
diff --git a/chrome/test/ui/npapi_test_helper.cc b/chrome/test/ui/npapi_test_helper.cc
index 142539c461f5bfe1fbfd0e7c6b488155ad1e328c..f8cc827c59af13e7054abb11a94672f43687af58 100644
--- a/chrome/test/ui/npapi_test_helper.cc
+++ b/chrome/test/ui/npapi_test_helper.cc
@@ -45,9 +45,11 @@ void NPAPITesterBase::SetUp() {
   test_plugin_path_ = plugins_directory.AppendASCII(test_plugin_name_);
 
   file_util::CreateDirectory(plugins_directory);
-  ASSERT_TRUE(file_util::CopyDirectory(plugin_src, test_plugin_path_, true))
-      << "Copy failed from " << plugin_src.value()
-      << " to " << test_plugin_path_.value();
+  if (!file_util::PathExists(test_plugin_path_)) {
+    ASSERT_TRUE(file_util::CopyDirectory(plugin_src, test_plugin_path_, true))
+        << "Copy failed from " << plugin_src.value()
+        << " to " << test_plugin_path_.value();
+  }
 #if defined(OS_MACOSX)
   // The plugins directory isn't read by default on the Mac, so it needs to be
   // explicitly registered.