Re-register 3-way synthetic Finch PartitionAllocEverywhereAndPCScan

This is a just-in-case CL. If things don't go as planned with this
week's Dev release, we can consider using the next week's Dev
for repeating the 3-way no-PAE/PAE/PAE+PCScan experiment.

Note there is a difference from how it was previously configured. The
old configuration would correctly report PCScan is always disabled
on 32-bit platforms, leading to 33/66/0 population split that it'd had
to be adjusted at release time (rather cumbersome and errorprone). The
new configuration lies that PCScan is still enabled thus keeping
33/33/33 split and allowing us to easily ignore the PCScan enabled
bucket when analyzing results.

Bug: 11297512, 1121427
Change-Id: I5843ed1e1dfaddd3a9eb891097096a41077b9483
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2617526
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Sébastien Marchand <sebmarchand@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841750}

chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.cc

@@ -558,6 +558,31 @@ void ChromeBrowserMainExtraPartsMetrics::PreBrowserStart() {
 #endif
   );
 
+#if defined(OS_WIN) && BUILDFLAG(USE_PARTITION_ALLOC_AS_MALLOC)
+  // Records whether or not PartitionAlloc-Everywhere is enabled, and whether
+  // PCScan is enabled on top of it. This is meant for a 3-way experiment with 2
+  // binaries:
+  // - binary A: deployed to 33% users, with PA-E and PCScan off.
+  // - binary B: deployed to 66% users, with PA-E on, half of which having
+  //             PCScan on
+  //
+  // NOTE, deliberately don't use ALLOW_PCSCAN which depends on bitness. In the
+  // 32-bit case, PCScan is always disabled, but we'll deliberately misrepresent
+  // it as enabled here (and later ignored when analyzing results), in order to
+  // keep each population at 33%.
+  ChromeMetricsServiceAccessor::RegisterSyntheticFieldTrial(
+      "PartitionAllocEverywhereAndPCScan",
+#if BUILDFLAG(USE_PARTITION_ALLOC_AS_MALLOC)
+      base::FeatureList::IsEnabled(
+          base::features::kPartitionAllocPCScanBrowserOnly)
+          ? "EnabledWithPCScan"
+          : "EnabledWithoutPCScan"
+#else
+      "Disabled"
+#endif  // BUILDFLAG(USE_PARTITION_ALLOC_AS_MALLOC)
+  );
+#endif  // defined(OS_WIN) && BUILDFLAG(USE_PARTITION_ALLOC_AS_MALLOC)
+
 #if defined(OS_WIN) && BUILDFLAG(USE_PARTITION_ALLOC_AS_MALLOC)
   // Records whether or not BackupRefPtr and/or PCScan is enabled. This is meant
   // for a 3-way experiment with 2 binaries: