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Deprecation warnings in third-party code are informational. When they arise, do
not silence them (since we treat warnings as errors, these were erroring).

Instead, allow the warnings to go through, as warnings.

BUG=none
R=sdefresne@chromium.org

Review-Url: https://codereview.chromium.org/2341963008
Cr-Commit-Position: refs/heads/master@{#419169}