[DawnLPM] Add size_t types

Change-Id: I05b6c8564ab8003b8312609e55dcadc26fb8a7d0
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/152460
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Loko Kung <lokokung@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Brendon Tiszka <tiszka@chromium.org>
diff --git a/generator/dawn_json_generator.py b/generator/dawn_json_generator.py
index a877e91..a210d0a 100644
--- a/generator/dawn_json_generator.py
+++ b/generator/dawn_json_generator.py
@@ -690,6 +690,7 @@
             "uint16_t": "uint32",
             "uint32_t": "uint32",
             "uint64_t": "uint64",
+            "size_t": "uint64",
         }
 
         assert typ in cpp_to_protobuf_type