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ProcessStatsStub is an interface to receive process resource usage information
from a ProcessStats.

ProcessResourceUsage is a structure to store the resource usage of one process.
AggregatedProcessResourceUsage is a structure to store the resource usgaes of
several processes.

ProcessStatsAgent is an interface to retrieve resource usage of one process.
CurrentProcessStatsAgent is a ProcessStatsAgent implementation to retrieve
resource usage from current process.
ForwardProcessStatsAgent receives resource usage from a third-party and forwards
the latest usage as a ProcessStatsAgent.

ProcessStatsSender is a class to regularly report resource usages from various
ProcessStatsAgent implementations to a ProcessStatsStub. It merges several
non-empty ProcessResourceUsage into one AggregatedProcessResourceUsage.
This is typical useful on Windows: we need to calculate the resource usages from
both network process and desktop process.

R=sergeyu@chromium.org, jamiewalch@chromium.org
BUG=650926

Review-Url: https://codereview.chromium.org/2775983003
Cr-Commit-Position: refs/heads/master@{#467753}