Add the EnabledWithThinLTO group to the ClangPGO study

This will let us use the Finch dashboards to evaluate the performance
gains of ThinLTO.

Bug: 1057737
Change-Id: I9ddaeb869a8aed369a7b5ca7af5994344dddf525
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2249085Reviewed-by: Hans Wennborg <hans@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Auto-Submit: Sébastien Marchand <sebmarchand@chromium.org>
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779779}

chrome/browser/BUILD.gn

@@ -72,6 +72,7 @@ buildflag_header("buildflags") {
   flags = [
     "CLANG_PGO=$chrome_pgo_phase",
     "ENABLE_KALEIDOSCOPE=$enable_kaleidoscope",
+    "USE_THIN_LTO=$use_thin_lto",
   ]
 }
 

chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.cc

@@ -530,7 +530,11 @@ void ChromeBrowserMainExtraPartsMetrics::PreBrowserStart() {
   // Log once here at browser start rather than at each renderer launch.
   ChromeMetricsServiceAccessor::RegisterSyntheticFieldTrial("ClangPGO",
 #if BUILDFLAG(CLANG_PGO)
+#if BUILDFLAG(USE_THIN_LTO)
+                                                            "EnabledWithThinLTO"
+#else
                                                             "Enabled"
+#endif
 #else
                                                             "Disabled"
 #endif