[ElementTiming] Only notify paint for loaded resources

This CL adds a check to ensure that an image is loaded before reporting
via the Element Timing API. We want to report the first paint but only
after the image is loaded. This matters for images that are
progressively rendered.

Bug: 879270
Change-Id: Ia0bcc68c1050967560103170be28c40238e839d9
Reviewed-on: https://chromium-review.googlesource.com/c/1415716
Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625327}
3 files changed