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This CL adds blendMode field to EffectPaintPropertyNode, upgrades
PaintPropertyTreeBuilder to create effect nodes if CSS mix-blend-mode is
applied to an element and ensure propery isolation on the parent, and adds
corresponding conversion in PaintArtifactCompositor.

BUG=609937
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2564193002
Cr-Commit-Position: refs/heads/master@{#438775}