memory coordinator: Purge memory under memory pressure

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 d4a2b12bda38119a8e1c13f61c8b201239c392a5..146f7f9eb5d00baf5d9cd2572f43eb94b6cfb40a 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -28514,6 +28514,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Memory.Experimental.Browser.PurgedMemory" units="MB">
+  <owner>bashi@chromium.org</owner>
+  <summary>
+    Amount of reclaimed memory (in terms of the private working set size) after
+    a purge request on the browser process. Recorded 2 seconds after the purge
+    request.
+  </summary>
+</histogram>
+
 <histogram base="true" name="Memory.Experimental.Renderer" units="MB">
 <!-- Name completed by histogram_suffixes name="RendererMemoryAllocator" -->
 
@@ -28530,6 +28539,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Memory.Experimental.Renderer.PurgedMemory" units="MB">
+  <owner>bashi@chromium.org</owner>
+  <summary>
+    Amount of reclaimed memory (in terms of
+    Memory.Experimental.Renderer.TotalAllocated) after a purge request on a
+    renderer process. Recorded 2 seconds after the purge request.
+  </summary>
+</histogram>
+
 <histogram name="Memory.Experimental.Renderer.Uptime" units="ms">
   <owner>keishi@chromium.org</owner>
   <summary>