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The RequestPicker is responsible for choosing which request from the
request queue to process next.  It will apply policies from the policy
class to the data returned by the request queue, pick a likely
candidate, and call back to the request coordinator so it can start
offlining the candidate.  If there is no candidate, it will call back
to the request coordinator to let it know processing is done for this
cycle.

BUG=610521

Review-Url: https://codereview.chromium.org/2020833002
Cr-Commit-Position: refs/heads/master@{#396967}