Add synthetic field trial group for Clang PGO

Bug: 1060025
Change-Id: I9e855b0f9290a0ac4785f88657d47d19ce250efb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2159533Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Cr-Commit-Position: refs/heads/master@{#761174}

chrome/browser/BUILD.gn

@@ -6,6 +6,7 @@ import("//base/allocator/allocator.gni")
 import("//build/buildflag_header.gni")
 import("//build/config/buildflags_paint_preview.gni")
 import("//build/config/chrome_build.gni")
+import("//build/config/compiler/pgo/pgo.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/jumbo.gni")
@@ -69,7 +70,10 @@ if (is_win) {
 
 buildflag_header("buildflags") {
   header = "buildflags.h"
-  flags = [ "ENABLE_KALEIDOSCOPE=$enable_kaleidoscope" ]
+  flags = [
+    "CLANG_PGO=$chrome_pgo_phase",
+    "ENABLE_KALEIDOSCOPE=$enable_kaleidoscope",
+  ]
 }
 
 # This proto library is used for non-android NTPs below.

chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.cc

@@ -25,8 +25,10 @@
 #include "build/build_config.h"
 #include "chrome/browser/about_flags.h"
 #include "chrome/browser/browser_process.h"
+#include "chrome/browser/buildflags.h"
 #include "chrome/browser/chrome_browser_main.h"
 #include "chrome/browser/metrics/bluetooth_available_utility.h"
+#include "chrome/browser/metrics/chrome_metrics_service_accessor.h"
 #include "chrome/browser/metrics/process_memory_metrics_emitter.h"
 #include "chrome/browser/shell_integration.h"
 #include "components/flags_ui/pref_service_flags_storage.h"
@@ -537,6 +539,15 @@ void ChromeBrowserMainExtraPartsMetrics::PreBrowserStart() {
   flags_ui::PrefServiceFlagsStorage flags_storage(
       g_browser_process->local_state());
   about_flags::RecordUMAStatistics(&flags_storage);
+
+  // Log once here at browser start rather than at each renderer launch.
+  ChromeMetricsServiceAccessor::RegisterSyntheticFieldTrial("ClangPGO",
+#if BUILDFLAG(CLANG_PGO)
+                                                            "Enabled"
+#else
+                                                            "Disabled"
+#endif
+  );
 }
 
 void ChromeBrowserMainExtraPartsMetrics::PostBrowserStart() {

chrome/browser/metrics/chrome_metrics_service_accessor.h

@@ -86,6 +86,8 @@ class ChromeMetricsServiceAccessor : public metrics::MetricsServiceAccessor {
   friend class ::CrashesDOMHandler;
   friend class ::FlashDOMHandler;
   friend class ChromeBrowserFieldTrials;
+  // For ClangPGO.
+  friend class ChromeBrowserMainExtraPartsMetrics;
   // For StackSamplingConfiguration.
   friend class ChromeBrowserMainParts;
   friend class ChromeContentBrowserClient;