[PartitionAlloc] Register synthetic Finch for GigaCage

Bug: 1086388
Change-Id: Ifd280c58451c2730ac0a844a9c5e02c41d0baaf2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626697
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Benoit L <lizeb@chromium.org>
Auto-Submit: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843256}

chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.cc

@@ -606,6 +606,11 @@ void ChromeBrowserMainExtraPartsMetrics::PreBrowserStart() {
 #endif
   );
 #endif  // defined(OS_WIN) && BUILDFLAG(USE_PARTITION_ALLOC_AS_MALLOC)
+
+  ChromeMetricsServiceAccessor::RegisterSyntheticFieldTrial(
+      "PartitionAllocGigaCageSynthetic",
+      base::features::IsPartitionAllocGigaCageEnabled() ? "Enabled"
+                                                        : "Disabled");
 }
 
 void ChromeBrowserMainExtraPartsMetrics::PostBrowserStart() {