Fixed leak on LRU snapshot cache implementation.
Fixed leak on LRU snapshot cache implementation.

Pinned snapshot images were leaked when receiving a memory warning.

BUG=None

Review URL: https://codereview.chromium.org/1687453002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#374669}

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