[AiS] Better prefetch cache.

Step 1 - factor out current cache and increase its size.

BUG=401758
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Review URL: https://codereview.chromium.org/455563002

Cr-Commit-Position: refs/heads/master@{#291307}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291307 0039d316-1c4b-4281-b951-d872f2087c98
8 files changed