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These APIs are intended to support spatial scheduling and task attribution.

Chromium implementation: https://codereview.chromium.org/1314903007/

BUG=510398

Review URL: https://codereview.chromium.org/1316233010

git-svn-id: svn://svn.chromium.org/blink/trunk@201978 bbb929c8-8fbe-4397-9dbb-9b2b20218538