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To preload image assets used by css, this CL add new BUILD.gn target
generating images into a js file under '/js/preload_images.js'. When
launching camera app, wait until images are loaded to show camera view.

Bug: b/172342439
Test: ./cca deploy <DUT>
Test: pass closure compiler
Test: Manually on DUT
Change-Id: I9ee8f73d0eed56c0002180259951235b6d0e642e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2531439
Commit-Queue: Shik Chen <shik@chromium.org>
Reviewed-by: Shik Chen <shik@chromium.org>
Auto-Submit: Inker Kuo <inker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828660}