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DOMNodeRemovedFromDocument as those are used extremely rarely.  We
don't have any data for how much of this usage is attributable to
extensions.  It will be interesting to watch this data as we
evangelize MutationObservers, especially among extension developers.

Another metric we have is for Blob.webkitSlice:

Ratio of Blob.webkitSlice calls to Blob.slice: 14.87%
Ratio of Blob.webkitSlice calls to Document creation: 0.0053%

It's difficult to know how to interpret this data because we don't
actually correlate calls to webkitSlice with Documents or Pages.
Instead, we just count the total number of calls across all Documents.
 This gives us an upper bound on how many Documents (or Pages) would
be affected by deleting Blob.webkitSlice, but doesn't measure that
information as accurately as the data we have for mutation events.

We are also gathering metrics on the usage of vendor prefixes in CSS
properties, but that histogram doesn't appear to have been in the
field long enough to have meaningful data.

Adam