Turn on computing img aspect ratios from width/height attributes

This turns on the flag to compute the aspect ratio for <img> elements
from the width/height attributes, if any, while the image is loading.

Intent to ship: https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/GePU9T8UpEc/bET6Mi60CgAJ

https://chromestatus.com/feature/5695266130755584

Bug: 979891
Change-Id: Ib7aa7c8766807613af2bf01eea21a64925095763
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1865546
Auto-Submit: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706673}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -146,7 +146,7 @@
       // from CSS.
       // https://github.com/WICG/intrinsicsize-attribute/issues/16
       name: "AspectRatioFromWidthAndHeight",
-      status: "test",
+      status: "stable",
     },
     {
       name: "AsyncClipboard",