Add the UMA metric Memory.Gpu.PhysicalFootprint.MacOS

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 6ad23e7aa6facea9ced1748ef02269d62f252277..5f6b2592f9e441c0c60f7ae34af9d68cf73ea07e 100644
--- a/chrome/browser/memory_details.h
+++ b/chrome/browser/memory_details.h
@@ -69,6 +69,9 @@ struct ProcessMemoryInformation {
   RendererProcessType renderer_type;
   // A collection of titles used, i.e. for a tab it'll show all the page titles.
   std::vector<base::string16> titles;
+  // The physical footprint is a macOS concept that tracks anonymous,
+  // non-discardable memory.
+  size_t phys_footprint;
 };
 
 typedef std::vector<ProcessMemoryInformation> ProcessMemoryInformationList;