[parkable_images] Add metrics for parkable images.

This CL adds metrics to allow us to track how many eligible images we have in renderers.

This requires keeping track of these images, which is done in this patch along with the metrics.

Bug: 1154295
Change-Id: I9c529f31b7d106741595afe57cd4c13d1f5606be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2564938


Commit-Queue: Thiabaud Engelbrecht <thiabaud@google.com>
Reviewed-by: Benoit L <lizeb@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835488}