Cleanup PreferenceMACs on browser_tests startup

chrome/test/base/chrome_test_launcher.cc

Index: chrome/test/base/chrome_test_launcher.cc
diff --git a/chrome/test/base/chrome_test_launcher.cc b/chrome/test/base/chrome_test_launcher.cc
index a6e39530d5e307530e1de9bd951a0a9369f1db67..10ef4ca77921c293af7ea3db6f551edeb54f4fc4 100644
--- a/chrome/test/base/chrome_test_launcher.cc
+++ b/chrome/test/base/chrome_test_launcher.cc
@@ -52,7 +52,9 @@
 #endif
 
 #if defined(OS_WIN)
+#include "base/win/registry.h"
 #include "chrome/app/chrome_crash_reporter_client_win.h"
+#include "chrome/install_static/install_util.h"
 #endif
 
 ChromeTestSuiteRunner::ChromeTestSuiteRunner() {}
@@ -96,6 +98,20 @@ ChromeTestLauncherDelegate::CreateContentMainDelegate() {
   return new ChromeMainDelegate();
 }
 
+void ChromeTestLauncherDelegate::PreSharding() {
+#if defined(OS_WIN)
+  // Pre-test cleanup for registry state keyed off the profile dir (which can
+  // proliferate with the use of uniquely named scoped_dirs):
+  // https://crbug.com/721245. This needs to be here in order not to be racy
+  // with any tests that will access that state.
+  base::win::RegKey key(HKEY_CURRENT_USER,
+                        install_static::GetRegistryPath().c_str(),
+                        KEY_SET_VALUE | KEY_QUERY_VALUE);
+  if (key.Valid())
+    key.DeleteKey(L"PreferenceMACs");
+#endif
+}
+
 int LaunchChromeTests(int default_jobs,
                       content::TestLauncherDelegate* delegate,
                       int argc,