Clarify the meaning of the 'v8 cache options' strings.
Clarify the meaning of the 'v8 cache options' strings.

BUG=b/22860790

Review URL: https://codereview.chromium.org/1404323002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#354306}

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