Rename DataReductionProxyUsageStats to DataReductionProxyBypassStats

As the comment in data_reduction_proxy_usage_stats.h by bengr@,
rename DataReductionProxyUsageStats to DataReductionProxyBypassStats
and related class. Rename the files also.

BUG=462846
R=bengr@chromium.org

Review URL: https://codereview.chromium.org/956223002

Cr-Commit-Position: refs/heads/master@{#323191}
19 files changed