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..d74756ef63764f0efe33b569075f0cee3081301a 100644
--- a/Source/wtf/Partitions.cpp
+++ b/Source/wtf/Partitions.cpp
@@ -72,6 +72,22 @@ void Partitions::shutdown()
     (void) m_fastMallocAllocator.shutdown();
 }
 
+void Partitions::releaseFreeMemory()
+{
+    ASSERT(isMainThread());
+
+    spinLockLock(&getBufferPartition()->lock);

[haraken, 2015/06/03 11:39:00]

Can we move this lock into partitionPurgeMemory?

We want to write these as:

  getBufferPartition()->partitionPurgeMemory();
  getFastMallocPartition()->partitionPurgeMemory();
  ...;

[ssid, 2015/06/03 11:45:31]

1. PartitionRootBase is a struct and partitionPurgeMemory() is a function no in
the struct.
2. Lock is only needed in PartitionRootGeneric and not in PartitionRoot (both of
these extend PartitionRootBase).

I would have refactor partitionPurgeMemory() to move into the struct and have
two different implementation for it, one with lock and one without.

WDYT I should do here?

+    partitionPurgeMemory(getBufferPartition());
+    spinLockUnlock(&getBufferPartition()->lock);
+
+    spinLockLock(&getFastMallocPartition()->lock);
+    partitionPurgeMemory(getFastMallocPartition());
+    spinLockUnlock(&getFastMallocPartition()->lock);
+
+    partitionPurgeMemory(getObjectModelPartition());
+    partitionPurgeMemory(getRenderingPartition());
+}
+
 void Partitions::reportMemoryUsageHistogram()
 {
     static size_t supportedMaxSizeInMB = 4 * 1024;