CodeGen: Make make_default_expr support buffer source types

Bug: 839389
Change-Id: Ic95b6207bfaccbe91c2165d09614138bed00ba68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2016776
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735327}

third_party/blink/renderer/bindings/scripts/bind_gen/blink_v8_bridge.py

@@ -259,6 +259,10 @@ def make_default_value_expr(idl_type, default_value):
         elif idl_type.unwrap().is_string:
             initializer = None  # String::IsNull() by default
             assignment_value = "String()"
+        elif idl_type.unwrap().is_buffer_source_type:
+            initializer = "nullptr"
+            is_initializer_lightweight = True
+            assignment_value = "nullptr"
         elif type_info.value_t == "ScriptValue":
             initializer = None  # ScriptValue::IsEmpty() by default
             assignment_value = "ScriptValue()"