Not sure why it wasn't reported here, but this has been committed here:
https://codereview.chromium.org/2514513004/

Your change meets the bar and is auto-approved for M56 (branch: 2924)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fa7af7cdfd43527f2e0d1e9059807dacb377be26

commit fa7af7cdfd43527f2e0d1e9059807dacb377be26
Author: Daniel Murphy <dmurph@chromium.org>
Date: Wed Nov 23 18:29:00 2016

[BlobStorage] Fix UMA reporting.

R=jsbell@chromium.org
BUG= 666862 

Review-Url: https://codereview.chromium.org/2514513004
Cr-Commit-Position: refs/heads/master@{#433760}
(cherry picked from commit 1e85b53656cc7053706890de27b3061a1390a153)

Review URL: https://codereview.chromium.org/2521723008 .

Cr-Commit-Position: refs/branch-heads/2924@{#79}
Cr-Branched-From: 3a87aecc31cd1ffe751dd72c04e5a96a1fc8108a-refs/heads/master@{#433059}

[modify] https://crrev.com/fa7af7cdfd43527f2e0d1e9059807dacb377be26/storage/browser/blob/blob_storage_context.cc
[modify] https://crrev.com/fa7af7cdfd43527f2e0d1e9059807dacb377be26/tools/metrics/histograms/histograms.xml

Thanks for the fix. Please tag it as fixed once all looks good.