Add a synthetic Finch trial for the segment heap

This synthetic Finch trial will allow measuring the impact that the
segment heap has on performance.

Bug: 1014701
Change-Id: I614f4b94fc11e43878d0d2f16af018490fc170f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2283241
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788339}

chrome/browser/BUILD.gn

@@ -45,6 +45,10 @@ if (is_android) {
   import("//tools/grit/grit_rule.gni")
 }
 
+if (is_win) {
+  import("//build/config/win/manifest.gni")
+}
+
 additional_modules_list_file =
     "$root_gen_dir/chrome/browser/internal/additional_modules_list.txt"
 
@@ -82,6 +86,9 @@ buildflag_header("buildflags") {
     "ENABLE_KALEIDOSCOPE=$enable_kaleidoscope",
     "USE_THIN_LTO=$use_thin_lto",
   ]
+  if (is_win) {
+    flags += [ "ENABLE_SEGMENT_HEAP=$enable_segment_heap" ]
+  }
 }
 
 # This proto library is used for non - android NTPs below.

chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.cc

@@ -549,6 +549,22 @@ void ChromeBrowserMainExtraPartsMetrics::PreBrowserStart() {
                                                             "Disabled"
 #endif
   );
+
+  // Records whether or not the Segment heap is in use.
+#if defined(OS_WIN)
+  if (base::win::GetVersion() >= base::win::Version::WIN10_20H1) {
+    ChromeMetricsServiceAccessor::RegisterSyntheticFieldTrial("WinSegmentHeap",
+#if BUILDFLAG(ENABLE_SEGMENT_HEAP)
+                                                              "OptedIn"
+#else
+                                                              "OptedOut"
+#endif
+    );
+  } else {
+    ChromeMetricsServiceAccessor::RegisterSyntheticFieldTrial("WinSegmentHeap",
+                                                              "NotSupported");
+  }
+#endif
 }
 
 void ChromeBrowserMainExtraPartsMetrics::PostBrowserStart() {