cc: Ensure to return pending now before active eventually tiles.

cc: Ensure to return pending now before active eventually tiles.

This patch is part 2 of a fix to return pending now tiles before active
eventually tiles. We already do this at a pair level. This patch ensures
that we also do it across pairs. That is, given two pairs of layers,
if the highest priority tile on one is an active eventually tile and
the highest priority tile on the other is a pending now tile, then
prefer the pending now tile (this is just for smoothness).

This ensures that we activate if we're prepainting too far.

BUG=426287
R=reveman

Review URL: https://codereview.chromium.org/675663002

Cr-Commit-Position: refs/heads/master@{#300981}