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To help guide development and track performance improvements in specific areas this new metric will compile the top-10 slowest tasks across all the pages in a page-set.

Part of the scheduling effort is to move long-running tasks into 'idle' periods or refactor them to make them more nibble-able. Knowing which tasks we need to concentrate on is the first step in this.

It is hoped that this code will also be built-on to create a metric to find out which long-running tasks most frequently block high priority tasks (e.g. Does GC often block user input being processed?) so we can move them out of the way.

BUG=421434

Review URL: https://codereview.chromium.org/636203002

Cr-Commit-Position: refs/heads/master@{#300084}