GitLab

Improves on element-id-to-layer-id followed by layer-id-to-node-index lookups.

BUG=674258
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2619383004
Cr-Commit-Position: refs/heads/master@{#442743}