content: Improve HostDiscardableSharedMemoryManager's memory pressure handling.

content/common/host_discardable_shared_memory_manager.cc

Index: content/common/host_discardable_shared_memory_manager.cc
diff --git a/content/common/host_discardable_shared_memory_manager.cc b/content/common/host_discardable_shared_memory_manager.cc
index c428bc5fe3473b7204ef87f8846f2aa45ac42c3c..fb71349c10477d4ffc2e5151586dc54de9d5d1f6 100644
--- a/content/common/host_discardable_shared_memory_manager.cc
+++ b/content/common/host_discardable_shared_memory_manager.cc
@@ -159,7 +159,10 @@ void HostDiscardableSharedMemoryManager::OnMemoryPressure(
 
   switch (memory_pressure_level) {
     case base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE:
+      break;
     case base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_MODERATE:
+      // Purge memory until usage is within half of |memory_limit_|.
+      ReduceMemoryUsageUntilWithinLimit(memory_limit_ / 2);
       break;
     case base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_CRITICAL:
       // Purge everything possible when pressure is critical.