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The simple cache can use too many FDs and crash systems with low FD
limits, like Macs. Longer term we should probably have ways to not
keep so many files open simultaneously, but the quick fix is to raise
the limit on Mac.

R=jkarlin@chromium.org,clamy@chromium.org
BUG=634413

Review-Url: https://codereview.chromium.org/2399073002
Cr-Commit-Position: refs/heads/master@{#423846}