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"Extensible" annotation has no effect for structs [1] and this CL
removes them in notifications.mojom.

[1] https://chromium.googlesource.com/chromium/src/+/master/mojo/public/tools/bindings/README.md#attributes

Bug: none
Test: none

Change-Id: Ie296e48b91c28d750bf5a1f6d18f1a696c8bc0a2
Reviewed-on: https://chromium-review.googlesource.com/c/1296569Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Commit-Queue: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602998}