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That event will contain ExecutionMode/SequenceToken and TaskPriority.

It would be better to tack an optional extra arg to the main task event but
tracing doesn't currently allow this (http://crbug.com/652692).

BUG=649084

Review-Url: https://codereview.chromium.org/2392903002
Cr-Commit-Position: refs/heads/master@{#423681}