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

third_party/sqlite/sqlite.gyp

Index: third_party/sqlite/sqlite.gyp
diff --git a/third_party/sqlite/sqlite.gyp b/third_party/sqlite/sqlite.gyp
index 160a4b27db3c27b6dd63a8eeb92df610e26263b7..e8ad2102d0048a85537d2f70928c20e5e1823b04 100644
--- a/third_party/sqlite/sqlite.gyp
+++ b/third_party/sqlite/sqlite.gyp
@@ -28,6 +28,12 @@
       # appropriately.  Chromium doesn't configure SQLite for that, and would
       # 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.