Add function to compute proportional set size for OS_WIN

base/process/process_metrics.h

Index: base/process/process_metrics.h
diff --git a/base/process/process_metrics.h b/base/process/process_metrics.h
index d9c9952201b816f8c1a28691a7e96add983378c3..2448a715ace9b5eade7f09524561ffcedb666cc3 100644
--- a/base/process/process_metrics.h
+++ b/base/process/process_metrics.h
@@ -145,6 +145,9 @@ class BASE_EXPORT ProcessMetrics {
   // usage in bytes, as per definition of WorkingSetBytes. Note that this
   // function is somewhat expensive on Windows (a few ms per process).
   bool GetWorkingSetKBytes(WorkingSetKBytes* ws_usage) const;
+  // Computes pss (proportional set size) of a process. Note that this
+  // function is somewhat expensive on Windows (a few ms per process).
+  bool GetProportionalSetSizeBytes(uint64_t* pss_bytes) const;
 
 #if defined(OS_MACOSX)
   // Fills both CommitedKBytes and WorkingSetKBytes in a single operation. This