Add comments for struct members that were changed from size_t to uin32_t or uint64_t for IPC safety.

components/autofill/core/common/form_field_data.h

Index: components/autofill/core/common/form_field_data.h
diff --git a/components/autofill/core/common/form_field_data.h b/components/autofill/core/common/form_field_data.h
index 03172390c5e1e24887bfb2dc65f19c31f8f643dc..c455871beb412124bba8247bf73c594b15d73bc6 100644
--- a/components/autofill/core/common/form_field_data.h
+++ b/components/autofill/core/common/form_field_data.h
@@ -45,6 +45,9 @@ struct FormFieldData {
   base::string16 value;
   std::string form_control_type;
   std::string autocomplete_attribute;
+  // Note: we use uint32_t instead of size_t because this struct is sent over
+  // IPC which could span 32 & 64 bit processes. This is fine since the length
+  // shouldn't exceed UINT32_MAX even on 64 bit builds.
   uint32_t max_length;
   bool is_autofilled;
   bool is_checked;