Add experimental private memory footprint metrics.

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 ba76f7beb6b4bdfe4607da6ff100bfc540735407..87f0d1dd16a5fa8dda76ad33cd213113e1bc5c8f 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -29520,6 +29520,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Memory.Experimental.Browser.PrivateMemoryFootprint.MacOS"
+    units="MB">
+  <owner>erikchen@chromium.org</owner>
+  <summary>
+    A rough estimate of the private memory footprint of the browser process.
+  </summary>
+</histogram>
+
 <histogram name="Memory.Experimental.Browser.PurgedMemory" units="MB">
   <owner>bashi@chromium.org</owner>
   <summary>
@@ -29546,6 +29554,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Memory.Experimental.Extension.PrivateMemoryFootprint.MacOS"
+    units="MB">
+  <owner>erikchen@chromium.org</owner>
+  <summary>
+    A rough estimate of the private memory footprint of an extension process.
+  </summary>
+</histogram>
+
 <histogram name="Memory.Experimental.Gpu.PhysicalFootprint.MacOS" units="MB">
   <owner>erikchen@chromium.org</owner>
   <summary>
@@ -29556,6 +29572,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Memory.Experimental.Gpu.PrivateMemoryFootprint.MacOS"
+    units="MB">
+  <owner>erikchen@chromium.org</owner>
+  <summary>
+    A rough estimate of the private memory footprint of the GPU process.
+  </summary>
+</histogram>
+
 <histogram base="true" name="Memory.Experimental.Renderer" units="MB">
 <!-- Name completed by histogram_suffixes name="RendererMemoryAllocator" -->
 
@@ -29572,6 +29596,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Memory.Experimental.Renderer.PrivateMemoryFootprint.MacOS"
+    units="MB">
+  <owner>erikchen@chromium.org</owner>
+  <summary>
+    A rough estimate of the private memory footprint of a renderer process.
+  </summary>
+</histogram>
+
 <histogram name="Memory.Experimental.Renderer.PurgedMemory" units="MB">
   <owner>bashi@chromium.org</owner>
   <summary>