[Reading mode] Download lazy images to the ax tree

Many websites use the loading=lazy attribute on their images. In this
case, the data url is not populated, but the regular src url is. If the
data url is null, try the regular url so that the accessibility tree
still gets the image information even if it's loaded lazily.

This does not fix the case where images are loaded lazily in a more
custom way (e.g. via javascript or the data-src attribute). That logic
is more complex and will require more investigation.

Bug: 435545158
Change-Id: Icb84bad9443fa356510ca3a757c39abf63db13e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6916349
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Kristi Saney <kristislee@google.com>
Auto-Submit: Kristi Saney <kristislee@google.com>
Cr-Commit-Position: refs/heads/main@{#1512778}