Expose ReleaseFreeMemory to free up memory on memory pressure or idle cleanup.

Source/wtf/Partitions.cpp

Index: Source/wtf/Partitions.cpp
diff --git a/Source/wtf/Partitions.cpp b/Source/wtf/Partitions.cpp
index 4eba302cc10cb0a1a53ae5be0717046b9547b9e8..a9b828dd22f3f91808fe1403b41c2a9a3c398f19 100644
--- a/Source/wtf/Partitions.cpp
+++ b/Source/wtf/Partitions.cpp
@@ -72,6 +72,16 @@ void Partitions::shutdown()
     (void) m_fastMallocAllocator.shutdown();
 }
 
+void Partitions::decommitFreeableMemory()
+{
+    ASSERT(isMainThread());
+
+    partitionPurgeMemoryGeneric(getBufferPartition());
+    partitionPurgeMemoryGeneric(getFastMallocPartition());
+    partitionPurgeMemory(getObjectModelPartition());
+    partitionPurgeMemory(getRenderingPartition());
+}
+
 void Partitions::reportMemoryUsageHistogram()
 {
     static size_t supportedMaxSizeInMB = 4 * 1024;