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This class has a test method that allows storing a closure to be called
when this ExtensionFunction is invoked. Since ExtensionFunctions should
only call Run() (at most) once, this Closure can be a OnceClosure.

We also clean it up to avoid using a pointer.

Bug: 1152268
Change-Id: Ie7ebc98ad8569ac31f8b6b553426c2bc40c3ea8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2602888Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840435}