Add temporary CHECK to investigate bad IPCs

BigBuffer intentionally constructs a malformed object when it can't
provide a suitable representation of its contents for transmission. This
is suspected to be the underlying cause for a few assorted bad IPC
reports.

This CL adds a temporary CHECK to investigate that hypothesis.

Bug: 1076341, 1087814
Tbr: dcheng@chromium.org
Change-Id: I2e07f3fcb9642034987e8bf3194b6817fbd701c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2231584Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#775304}

mojo/public/cpp/base/big_buffer.cc

@@ -67,6 +67,10 @@ void TryCreateSharedMemory(
       // instead produce an invalid buffer. This will always fail validation on
       // the receiving end.
       *storage_type = BigBuffer::StorageType::kInvalidBuffer;
+
+      // TODO(crbug.com/1076341): Remove this temporary CHECK to investigate
+      // some bad IPC reports likely caused by this path.
+      CHECK(false);
       return;
     }
   }