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This uses an estimated GPU utilization ratio from ArCoreGl. The goal is
to reduce the recommended viewport scale if the web app is unable to hit
a target framerate due to excessive GPU work, but not do so if it's slow
due to excessive JS computations or similar where reducing the viewport
wouldn't help. This is opt-in, the scale only takes effect if the app
calls view.requestViewportScale(view.recommendedViewportScale) followed
by xrwebgllayer.getViewport(view).

(Dynamic viewport scaling is only available if WebXRIncubations is
enabled.)

Spec link for context:
https://immersive-web.github.io/webxr/#dom-xrview-recommendedviewportscale

Change-Id: I0a8598d8bd03f27fd1d2cc6c542ff79812825d74
Bug: 1133381
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2213113
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Reviewed-by: Piotr Bialecki <bialpio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816813}