Add CHECKs to help debug crashes exposed by adding a member to IPC::Message::Header.

content/browser/loader/async_resource_handler.cc

Index: content/browser/loader/async_resource_handler.cc
diff --git a/content/browser/loader/async_resource_handler.cc b/content/browser/loader/async_resource_handler.cc
index 2eace36852ea9a048c8a6dd6f77135619b42752a..78a6e8944c8b7687b2b187cae50e2fc72d2c31d9 100644
--- a/content/browser/loader/async_resource_handler.cc
+++ b/content/browser/loader/async_resource_handler.cc
@@ -329,6 +329,9 @@ bool AsyncResourceHandler::OnReadCompleted(int bytes_read, bool* defer) {
   int encoded_data_length = current_transfer_size - reported_transfer_size_;
   reported_transfer_size_ = current_transfer_size;
 
+  // TODO(erikchen): Temporary debugging for http://crbug.com/527588.
+  CHECK_LE(data_offset, kBufferSize);
+
   filter->Send(new ResourceMsg_DataReceived(
       GetRequestID(), data_offset, bytes_read, encoded_data_length));
   ++pending_data_count_;