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This CL adds support to observe the first time an <img> element paints
via the PerformanceObserver (no support for performance timeline yet)
and under a flag that is disabled by default.

Implementation proposal:
https://docs.google.com/document/d/1BvHknbj3T-fUuj4RxHF8qGRNxbswtBqUjKiPx7iLXCc

Intent to Implement:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/2twP4Xdd1VI

Bug: 879270
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I715bd71be088f8055ece493db208caa9b97d736b
Reviewed-on: https://chromium-review.googlesource.com/1197349
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589712}