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The current mechanism is to create an instance of DataReductionProxyParams
inside of page_load_histogram, and to then check the proxy server inside of
DocumentState against DataReductionProxyParams. However, the plan is to make
the configuration dynamic, so we can no longer use DataReductionProxyParams.

BUG=452773

Review URL: https://codereview.chromium.org/966443002

Cr-Commit-Position: refs/heads/master@{#318917}