Design doc: https://docs.google.com/document/d/1USTH2Vd4D2tALsvZvy4B2aWotKWjkCYP5m0g7b90RAU/edit#

Milestone 1 prototype CL: https://chromium-review.googlesource.com/c/chromium/src/+/1251465

This bug tracks creating a placeholder 'DeferredImage' instead of calling CSS Paint, and plumbing the related information (just an id) through to the cc side. This was largely done during the milestone 1 prototype CL[0][1][2], and the main work remaining is to tidy it up and make sure we are architecturally sound.

[0]: https://chromium-review.googlesource.com/c/chromium/src/+/1251465/7/third_party/blink/renderer/platform/graphics/deferred_paint_image.h

[1]: https://chromium-review.googlesource.com/c/chromium/src/+/1251465/7/third_party/blink/renderer/core/css/css_paint_value.cc

[2]: https://chromium-review.googlesource.com/c/chromium/src/+/1251465/7/cc/paint/paint_image.h