Add experimental private memory footprint metrics.

chrome/browser/memory_details.h

Index: chrome/browser/memory_details.h
diff --git a/chrome/browser/memory_details.h b/chrome/browser/memory_details.h
index 5f6b2592f9e441c0c60f7ae34af9d68cf73ea07e..b0f84af9427db03bf5aee1b6a9edd7e26c50c589 100644
--- a/chrome/browser/memory_details.h
+++ b/chrome/browser/memory_details.h
@@ -72,6 +72,9 @@ struct ProcessMemoryInformation {
   // The physical footprint is a macOS concept that tracks anonymous,
   // non-discardable memory.
   size_t phys_footprint;
+  // TODO(erikchen): Remove this temporary estimate for private memory once the
+  // memory infra service emits the same metric. https://crbug.com/720541.
+  size_t private_memory_footprint;
 };
 
 typedef std::vector<ProcessMemoryInformation> ProcessMemoryInformationList;