Rename DataReductionProxyStatisticsPrefs to DataReductionProxyCompressionStats

As the comment in data_reduction_proxy_statistics_prefs.h by bengr@,
rename DataReductionProxyStatisticsPrefs to DataReductionProxyCompressionStats
and related class. Rename the files also.

BUG=462845
R=bengr@chromium.org, jam@chromium.org

Review URL: https://codereview.chromium.org/949533004

Cr-Commit-Position: refs/heads/master@{#323421}
26 files changed