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In preparation for moving metricsPrivate to //extensions, re-generate the
externs file and fix outdated types.

BUG=647397
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2346683004
Cr-Commit-Position: refs/heads/master@{#419120}