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This changes the opacity 0 paints that are ignored by the LCP algorithm.
Before, paints that would later be composited were not ignored, which
resulted in an inaccurate metric due to detecting LCP too early. After
this change, even will-change opacity paints are ignored, which could
result in elements not becoming candidates because they are never
repainted. In the special case where documentElement changes its
opacity, we consider the largest content that becomes visible as a valid
LCP candidate.

In other words this CL does the following 3 changes:
1) Ignores all LCP signals under style.opacity: 0, as before styles with
'will-change: opacity' were not ignored.
2) Saves off the LCP for ignored LCP signals if they are under
non-nested style.opacity = 0.
3.) If the document opacity style changes from opacity: 0 to non-zero,
we record the saved off LCP signal.

Bug: 1092473
Change-Id: I2a970a34fdb1db6c7c746060c286077553b816a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2316788
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794689}