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

third_party/sqlite/BUILD.gn

Index: third_party/sqlite/BUILD.gn
diff --git a/third_party/sqlite/BUILD.gn b/third_party/sqlite/BUILD.gn
index 0daa5ee69daa7ae7d4315a5298673759152791dd..76b92153f2292eeacdcfb864fe074e35af5c05c7 100644
--- a/third_party/sqlite/BUILD.gn
+++ b/third_party/sqlite/BUILD.gn
@@ -67,6 +67,13 @@ if (!use_system_sqlite) {
       # prefer to control distribution to worker threads.
       "SQLITE_MAX_WORKER_THREADS=0",
 
+      # Allow 256MB mmap footprint per connection.  Should not be too open-ended
+      # as that could cause memory fragmentation.  50MB encompasses the 99th
+      # percentile of Chrome databases in the wild.
+      # TODO(shess): A 64-bit-specific value could be 1G or more.
+      # TODO(shess): Figure out if exceeding this is costly.
+      "SQLITE_MAX_MMAP_SIZE=268435456",
+
       # Use a read-only memory map when mmap'ed I/O is enabled to prevent memory
       # stompers from directly corrupting the database.
       # TODO(shess): Upstream the ability to use this define.