Remove PaintInfo's paintingRoot

      PaintInfo had a paintingRoot member for supporting drawing a single
layout object (including descendants). This complexity is spread
throughout the painting code and only exists to support drag and drop
drag images:
https://chromium.googlesource.com/chromium/src/+/65e7e9d4860e50bb677228f0b83848cce208eb76

This patch switches to directly painting LayoutObjects and removes the
complex paintingRoot code.

ClusterTelemetry results: 0.5% record time savings:
https://storage.cloud.google.com/cluster-telemetry/tasks/chromium_perf_runs/pdr-20160118203222/html/index.html

BUG=577004

Review URL: https://codereview.chromium.org/1602513004

Cr-Commit-Position: refs/heads/master@{#370334}