The bug here is because of some confusion around behavior of BlobStorageContext::BuildBlob: when building a blob synchronously fails it does call its ready-for-transport callback with the failed status, but if building finishes synchronously it doesn't call the callback at all.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a3c3176337d980e3787a2f4e55b0659a38427330

commit a3c3176337d980e3787a2f4e55b0659a38427330
Author: Marijn Kruisselbrink <mek@chromium.org>
Date: Fri Aug 04 23:40:13 2017

Fix bug where sometimes BlobUnderConstruction instances could leak.

Bug:  752639 
Change-Id: Id57bfbaecebb54522767c1044be76119d4840ae5
Reviewed-on: https://chromium-review.googlesource.com/602875
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#492175}
[modify] https://crrev.com/a3c3176337d980e3787a2f4e55b0659a38427330/storage/browser/blob/blob_registry_impl.cc
[modify] https://crrev.com/a3c3176337d980e3787a2f4e55b0659a38427330/storage/browser/blob/blob_registry_impl.h
[modify] https://crrev.com/a3c3176337d980e3787a2f4e55b0659a38427330/storage/browser/blob/blob_registry_impl_unittest.cc

Apologies, applied the wrong component in bulk.

Setting Internals>Services>Storage to all children of issue 611935