Add a histogram for total private memory footprint.

chrome/browser/metrics/process_memory_metrics_emitter.cc

Index: chrome/browser/metrics/process_memory_metrics_emitter.cc
diff --git a/chrome/browser/metrics/process_memory_metrics_emitter.cc b/chrome/browser/metrics/process_memory_metrics_emitter.cc
index 6cf44b923b715d72f5fcaf5806691857380f1258..8f6e77c27258a020f209fa70c4a3f68abe39acc4 100644
--- a/chrome/browser/metrics/process_memory_metrics_emitter.cc
+++ b/chrome/browser/metrics/process_memory_metrics_emitter.cc
@@ -83,7 +83,9 @@ void ProcessMemoryMetricsEmitter::ReceivedMemoryDump(
   if (!ptr)
     return;
 
+  uint32_t private_footprint_total_kb = 0;
   for (const ProcessMemoryDumpPtr& pmd : ptr->process_dumps) {
+    private_footprint_total_kb += pmd->private_footprint;
     switch (pmd->process_type) {
       case memory_instrumentation::mojom::ProcessType::BROWSER:
         EmitBrowserMemoryMetrics(pmd);
@@ -100,4 +102,7 @@ void ProcessMemoryMetricsEmitter::ReceivedMemoryDump(
         break;
     }
   }
+  UMA_HISTOGRAM_MEMORY_LARGE_MB(
+      "Memory.Experimental.Total2.PrivateMemoryFootprint",
+      private_footprint_total_kb / 1024);
 }