Add headers-only component to DiskCachePerfTest.
Add headers-only component to DiskCachePerfTest.

Reading only the headers is a significant gap between SimpleCache and
the blockfile backend. Let's test that independently of reading the
whole bodies.

R=juliatuttle@chromium.org
BUG=611464

Review-Url: https://codereview.chromium.org/1976453003
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#393595}

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