Move purging memory from OnMemoryStateChange() to OnPurgeMemory() in discardable_memory

components/discardable_memory/service/discardable_shared_memory_manager.cc

Index: components/discardable_memory/service/discardable_shared_memory_manager.cc
diff --git a/components/discardable_memory/service/discardable_shared_memory_manager.cc b/components/discardable_memory/service/discardable_shared_memory_manager.cc
index a974b2d9e27d8f35d1a59a9fcc7a5e8fdfc6e0e0..e2966c798c78bfafbc28f69e0d85095aa9f89582 100644
--- a/components/discardable_memory/service/discardable_shared_memory_manager.cc
+++ b/components/discardable_memory/service/discardable_shared_memory_manager.cc
@@ -405,12 +405,15 @@ size_t DiscardableSharedMemoryManager::GetBytesAllocated() {
 
 void DiscardableSharedMemoryManager::OnMemoryStateChange(
     base::MemoryState state) {
+  // Don't use SetMemoryLimit() as it frees up existing allocations.
+  // OnPurgeMemory() is called to actually free up memory.
+  base::AutoLock lock(lock_);
   switch (state) {
     case base::MemoryState::NORMAL:
-      SetMemoryLimit(default_memory_limit_);
+      memory_limit_ = default_memory_limit_;
       break;
     case base::MemoryState::THROTTLED:
-      SetMemoryLimit(0);
+      memory_limit_ = 0;
       break;
     case base::MemoryState::SUSPENDED:
     // Note that SUSPENDED never occurs in the main browser process so far.
@@ -421,6 +424,11 @@ void DiscardableSharedMemoryManager::OnMemoryStateChange(
   }
 }
 
+void DiscardableSharedMemoryManager::OnPurgeMemory() {
+  base::AutoLock lock(lock_);
+  ReduceMemoryUsageUntilWithinLimit(0);
+}
+
 void DiscardableSharedMemoryManager::AllocateLockedDiscardableSharedMemory(
     int client_id,
     size_t size,