Make HistoryBackend a client of memory coordinator

components/history/core/browser/history_backend.h

Index: components/history/core/browser/history_backend.h
diff --git a/components/history/core/browser/history_backend.h b/components/history/core/browser/history_backend.h
index aeecc96ab01663b118f0bcafaa8501c00f830d13..64c2b62c9ec1530251dad47710acc1763c947fdb 100644
--- a/components/history/core/browser/history_backend.h
+++ b/components/history/core/browser/history_backend.h
@@ -32,6 +32,7 @@
 #include "components/history/core/browser/keyword_id.h"
 #include "components/history/core/browser/thumbnail_database.h"
 #include "components/history/core/browser/visit_tracker.h"
+#include "components/memory_coordinator/common/memory_coordinator_client.h"
 #include "sql/init_status.h"
 
 class HistoryURLProvider;
@@ -102,7 +103,8 @@ class QueuedHistoryDBTask {
 // functions in the history service. These functions are not documented
 // here, see the history service for behavior.
 class HistoryBackend : public base::RefCountedThreadSafe<HistoryBackend>,
-                       public HistoryBackendNotifier {
+                       public HistoryBackendNotifier,
+                       public memory_coordinator::MemoryCoordinatorClient {
  public:
   // Interface implemented by the owner of the HistoryBackend object. Normally,
   // the history service implements this to send stuff back to the main thread.
@@ -478,6 +480,9 @@ class HistoryBackend : public base::RefCountedThreadSafe<HistoryBackend>,
 
   base::Time GetFirstRecordedTimeForTest() { return first_recorded_time_; }
 
+  // memory_coordinator::MemoryCoordinatorClient implementation:
+  void OnMemoryStateChange(memory_coordinator::MemoryState state) override;
+
  protected:
   ~HistoryBackend() override;
 
@@ -579,6 +584,9 @@ class HistoryBackend : public base::RefCountedThreadSafe<HistoryBackend>,
   // Does the work of Init.
   void InitImpl(const HistoryDatabaseParams& history_database_params);
 
+  // Asks database to trim memory.
+  void TrimMemory(bool trim_aggressively);
+
   // Called when the system is under memory pressure.
   void OnMemoryPressure(
       base::MemoryPressureListener::MemoryPressureLevel memory_pressure_level);