Don't report 'custom' processes as untracked.

content/browser/browser_child_process_host_impl.cc

Index: content/browser/browser_child_process_host_impl.cc
diff --git a/content/browser/browser_child_process_host_impl.cc b/content/browser/browser_child_process_host_impl.cc
index 47cfea1b6a502bafce526ec00c9e9f7d915bc4d7..15f619a089d7b7ba5f2ebaeed8e229b9a6e4693f 100644
--- a/content/browser/browser_child_process_host_impl.cc
+++ b/content/browser/browser_child_process_host_impl.cc
@@ -499,9 +499,13 @@ void BrowserChildProcessHostImpl::CreateMetricsAllocator() {
       break;
 
     default:
-      UMA_HISTOGRAM_ENUMERATION(
-          "UMA.SubprocessMetricsProvider.UntrackedProcesses",
-          data_.process_type, PROCESS_TYPE_CONTENT_END);
+      // Report new processes. "Custom" ones are renumbered to 1000+ so that
+      // they won't conflict with any standard ones in the future.
+      int process_type = data_.process_type;
+      if (process_type >= PROCESS_TYPE_CONTENT_END)
+        process_type += 1000 - PROCESS_TYPE_CONTENT_END;
+      UMA_HISTOGRAM_SPARSE_SLOWLY(
+          "UMA.SubprocessMetricsProvider.UntrackedProcesses", process_type);
       return;
   }