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This CL adds support for WPR and network throttling (through WPR) for
testing the speculative_prefetch_predictor. The modifications are:
- Generate a WPR archive in generate_database.py
- Use the archive and set up throttling in prefetch_benchmark.

Also adds the ability to test several configuration of the predictor in
prefetch_benchmark, and make it loop.

BUG=655980

Review-Url: https://codereview.chromium.org/2561353002
Cr-Commit-Position: refs/heads/master@{#439131}