[sql] Use memory-mapped I/O for sql::Connection.

sql/connection.h

Index: sql/connection.h
diff --git a/sql/connection.h b/sql/connection.h
index 19592d9f0e44ed68764b9d25a88d9a65424ba9b7..0263bb1cedce5c3a220e643d8b786536f7214ec5 100644
--- a/sql/connection.h
+++ b/sql/connection.h
@@ -262,6 +262,7 @@ class SQL_EXPORT Connection {
   // true, this function will try to free all of the cache memory it can. If
   // |aggressively| is false, this function will try to cut cache memory
   // usage by half.
+  // NOTE(shess): See also ReleaseCacheMemory().
   void TrimMemory(bool aggressively);
 
   // Raze the database to the ground.  This approximates creating a
@@ -638,6 +639,12 @@ class SQL_EXPORT Connection {
     return clock_->Now();
   }
 
+  // Release page-cache memory if memory-mapped I/O is enabled and the database
+  // was changed.  Passing true for |assume_changed| allows overriding the
+  // change detection for cases like DDL (CREATE, DROP, etc), which do not
+  // participate in the total-rows-changed tracking.
+  void ReleaseCacheMemoryIfNeeded(bool assume_changed);
+
   // The actual sqlite database. Will be NULL before Init has been called or if
   // Init resulted in an error.
   sqlite3* db_;
@@ -679,6 +686,14 @@ class SQL_EXPORT Connection {
   // databases.
   bool poisoned_;
 
+  // |true| if SQLite memory-mapped I/O is enabled for this connection.
+  // Used by ReleaseCacheMemoryIfNeeded().
+  bool mmap_enabled_;
+
+  // Used by ReleaseCacheMemoryIfNeeded() to track if new changes have happened
+  // since memory was last released.
+  int total_changes_;
+
   ErrorCallback error_callback_;
 
   // Tag for auxiliary histograms.