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XRPlaneSet is a readonly setlike<XRPlane>.

Change-Id: Ifefac1ff5f04a5d52dfb80828b2a2c769c62ebf8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1663456
Commit-Queue: Piotr Bialecki <bialpio@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670328}