Update the .pak file format to version 2: Make resource ids 16bit

tools/grit/grit/format/data_pack.py

Index: tools/grit/grit/format/data_pack.py
diff --git a/tools/grit/grit/format/data_pack.py b/tools/grit/grit/format/data_pack.py
index c8a46ad1963d2419af45d0ea2f8bb41c2ac08696..112c13e2834ffe44da88cdbb7aab68bd59a529c0 100755
--- a/tools/grit/grit/format/data_pack.py
+++ b/tools/grit/grit/format/data_pack.py
@@ -15,7 +15,7 @@ from grit.node import message
 from grit.node import misc
 
 
-PACK_FILE_VERSION = 1
+PACK_FILE_VERSION = 2
 
 
 class DataPack(interface.ItemFormatter):
@@ -60,12 +60,13 @@ class DataPack(interface.ItemFormatter):
     ret.append(struct.pack("<II", PACK_FILE_VERSION, len(ids)))
     HEADER_LENGTH = 2 * 4             # Two uint32s.
 
-    index_length = len(ids) * 3 * 4   # Each entry is 3 uint32s.
+    # Each entry is 1 uint16 + 2 uint32s.
+    index_length = len(ids) * (2 + 2 * 4)
 
     # Write index.
     data_offset = HEADER_LENGTH + index_length
     for id in ids:
-      ret.append(struct.pack("<III", id, data_offset, len(resources[id])))
+      ret.append(struct.pack("<HII", id, data_offset, len(resources[id])))
       data_offset += len(resources[id])
 
     # Write data.