Add SwapMetricsObserverMac

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 7fd1572246cf423df6a3c9db4ce0ddf2d6d5cf01..f33d38a07fc62af0568d0067690743434945a50b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -29416,6 +29416,23 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Memory.Experimental.CompressedPagesPerSecond" units="pages/s">
+  <owner>bashi@chromium.org</owner>
+  <summary>
+    The number of pages compressed per second. Recorded every 60 seconds. Only
+    recorded on macOS.
+  </summary>
+</histogram>
+
+<histogram name="Memory.Experimental.DecompressedPagesPerSecond"
+    units="pages/s">
+  <owner>bashi@chromium.org</owner>
+  <summary>
+    The number of pages decompressed per second. Recorded every 60 seconds. Only
+    recorded on macOS.
+  </summary>
+</histogram>
+
 <histogram name="Memory.Experimental.Gpu.PhysicalFootprint.MacOS" units="MB">
   <owner>erikchen@chromium.org</owner>
   <summary>