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Comment had a 'not' which was likely a mistake, the Add*Attribute methods all
remove the attribute if it already exists before adding.

R=aboxhall,dmazzoni

Change-Id: Iaf0bc259ba8c6e9f64f4b443935ff01979ca893d
Reviewed-on: https://chromium-review.googlesource.com/c/1485613
Auto-Submit: Chris Hall <chrishall@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635159}