Decommit unused system pages when invoking WTF::DecommitSystemPages().

third_party/WebKit/Source/platform/wtf/allocator/Partitions.cpp

Index: third_party/WebKit/Source/platform/wtf/allocator/Partitions.cpp
diff --git a/third_party/WebKit/Source/platform/wtf/allocator/Partitions.cpp b/third_party/WebKit/Source/platform/wtf/allocator/Partitions.cpp
index 9a8ea36d4de2de432e8c6f0245a61b6904d00e72..b96cc6e8a005290b0454c61cd11a3dd12e4bb188 100644
--- a/third_party/WebKit/Source/platform/wtf/allocator/Partitions.cpp
+++ b/third_party/WebKit/Source/platform/wtf/allocator/Partitions.cpp
@@ -70,13 +70,17 @@ void Partitions::DecommitFreeableMemory() {
     return;
 
   PartitionPurgeMemoryGeneric(ArrayBufferPartition(),
-                              base::PartitionPurgeDecommitEmptyPages);
+                              base::PartitionPurgeDecommitEmptyPages |
+                                  base::PartitionPurgeDiscardUnusedSystemPages);
   PartitionPurgeMemoryGeneric(BufferPartition(),
-                              base::PartitionPurgeDecommitEmptyPages);
+                              base::PartitionPurgeDecommitEmptyPages |
+                                  base::PartitionPurgeDiscardUnusedSystemPages);
   PartitionPurgeMemoryGeneric(FastMallocPartition(),
-                              base::PartitionPurgeDecommitEmptyPages);
+                              base::PartitionPurgeDecommitEmptyPages |
+                                  base::PartitionPurgeDiscardUnusedSystemPages);
   PartitionPurgeMemory(LayoutPartition(),
-                       base::PartitionPurgeDecommitEmptyPages);
+                       base::PartitionPurgeDecommitEmptyPages |
+                           base::PartitionPurgeDiscardUnusedSystemPages);
 }
 
 void Partitions::ReportMemoryUsageHistogram() {