base: Add soft memory limit to DiscardableMemoryManager.

base/memory/discardable_memory_mac.cc

Index: base/memory/discardable_memory_mac.cc
diff --git a/base/memory/discardable_memory_mac.cc b/base/memory/discardable_memory_mac.cc
index b2982fc5d236a8b0aadb91b909d8e2c54160cb64..b2184e7d589c415f5cd98f09b0e77786d42333a2 100644
--- a/base/memory/discardable_memory_mac.cc
+++ b/base/memory/discardable_memory_mac.cc
@@ -25,7 +25,11 @@ namespace {
 const size_t kMacMemoryLimit = 512 * 1024 * 1024;
 
 struct SharedState {
-  SharedState() : manager(kMacMemoryLimit, kMacMemoryLimit) {}
+  SharedState()
+      : manager(kMacMemoryLimit,
+                kMacMemoryLimit,
+                kMacMemoryLimit,
+                TimeDelta::Max()) {}
 
   internal::DiscardableMemoryManager manager;
 };
@@ -166,6 +170,11 @@ void DiscardableMemory::UnregisterMemoryPressureListeners() {
 }
 
 // static
+bool DiscardableMemory::ReduceMemoryUsage() {
+  return internal::DiscardableMemoryEmulated::ReduceMemoryUsage();
+}
+
+// static
 void DiscardableMemory::GetSupportedTypes(
     std::vector<DiscardableMemoryType>* types) {
   const DiscardableMemoryType supported_types[] = {