memory metrics: record growth after purging BG renderer's cache

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 e508c28079a10c0d90d69950546a0d65d893f624..4c66a20b20093633985c8844bbee37831378e303 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -51852,6 +51852,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram base="true" name="PurgeAndSuspend.Experimental.MemoryGrowth"
+    units="KB">
+<!-- Name completed by histogram_suffixes name="PurgeAndSuspendExperiment" -->
+
+  <owner>tasak@google.org</owner>
+  <summary>
+    The growth of the background renderer's memory usage after a background
+    renderer is purged. This is recorded 5, 10 and 15 minutes after the purge.
+  </summary>
+</histogram>
+
 <histogram name="PurgeAndSuspend.Memory.BlinkGCKB" units="KB">
   <owner>tasak@google.com</owner>
   <summary>
@@ -115914,6 +115925,20 @@ value.
   <affected-histogram name="PLT.BeginToFinish_NormalLoad"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="PurgeAndSuspendExperiment" separator=".">
+  <suffix name="BlinkGCKB" label="Constrained to the BlinkGC allocator"/>
+  <suffix name="DiscardableKB" label="Constrained to discardable memory usage"/>
+  <suffix name="PartitionAllocKB"
+      label="Constrained to PartitionAlloc allocator"/>
+  <suffix name="MallocKB" label="Constrained to malloc memory usage"/>
+  <suffix name="TotalAllocatedKB"
+      label="Summed over the PartitionAlloc, malloc, discardable memory,
+             mainThreadIsolate() and BlinkGC allocators"/>
+  <suffix name="V8MainThreadIsolateKB"
+      label="Constrained to memory usage by mainThreadIsolate()"/>
+  <affected-histogram name="PurgeAndSuspend.Experimental.MemoryGrowth"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="QueryTimeSuffix" separator=".">
   <suffix name="0" label="N = 0"/>
   <suffix name="1" label="N = 1"/>