Add MemoryPurgeController.ReclaimedPartitionAllocInactiveTab

This UMA reports how much memory did MemoryPurgeController reclaim.

BUG=519146

Review URL: https://codereview.chromium.org/1460603002

Cr-Commit-Position: refs/heads/master@{#361284}

third_party/WebKit/Source/platform/MemoryPurgeController.cpp

@@ -45,7 +45,17 @@ void MemoryPurgeController::pageBecameInactive()
 
 void MemoryPurgeController::pageInactiveTask(Timer<MemoryPurgeController>*)
 {
+    static const size_t maxSizeInKB = 10 * 1024;
+
+    size_t totalSizeBefore = WTF::Partitions::totalSizeOfCommittedPages();
     purgeMemory(MemoryPurgeMode::InactiveTab);
+    size_t totalSizeAfter = WTF::Partitions::totalSizeOfCommittedPages();
+    if (totalSizeAfter >= totalSizeBefore)
+        return;
+    size_t reclaimedInKB = (totalSizeBefore - totalSizeAfter) / 1024 + 1;
+    if (reclaimedInKB >= maxSizeInKB)
+        reclaimedInKB = maxSizeInKB - 1;
+    Platform::current()->histogramCustomCounts("MemoryPurgeController.ReclaimedPartitionAllocInactiveTab", reclaimedInKB, 1, maxSizeInKB, 50);
 }
 
 void MemoryPurgeController::purgeMemory(MemoryPurgeMode purgeMode)

tools/metrics/histograms/histograms.xml

@@ -20129,6 +20129,12 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="MemoryPurgeController.ReclaimedPartitionAllocInactiveTab"
+    units="KB">
+  <owner>bashi@chromium.org</owner>
+  <summary>The amount of reclaimed memory after a tab became inactive.</summary>
+</histogram>
+
 <histogram name="MemoryWarning.EvictedTabTimeSinceActive" units="ms">
   <owner>lliabraa@chromium.org</owner>
   <summary>