win: Register synthetic field trial for clang builds.

chrome/browser/chrome_browser_main.cc

Index: chrome/browser/chrome_browser_main.cc
diff --git a/chrome/browser/chrome_browser_main.cc b/chrome/browser/chrome_browser_main.cc
index e1e2512423a569f65eec092b9d884e61dc3abc1d..04b027ab6d228fd98c578023c9c9516309439d34 100644
--- a/chrome/browser/chrome_browser_main.cc
+++ b/chrome/browser/chrome_browser_main.cc
@@ -1354,6 +1354,16 @@ void ChromeBrowserMainParts::PreBrowserStart() {
   SetupSyzyASAN();
 #endif
 
+#if defined(OS_WIN)
+  ChromeMetricsServiceAccessor::RegisterSyntheticFieldTrial("ChromeWinClang",
+#if defined(__clang__)
+                                                            "Enabled"
+#else
+                                                            "Disabled"
+#endif
+                                                            );
+#endif
+
 // Start the tab manager here so that we give the most amount of time for the
 // other services to start up before we start adjusting the oom priority.
 #if defined(OS_WIN) || defined(OS_MACOSX) || defined(OS_LINUX)