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Adds a DebugInfo message to the DistilledPageProto. The data for this message
is copied from the DomDistillerResult proto DebugInfo.

For now, only a field for the distiller log exists.

Depends on https://codereview.chromium.org/493853006/

BUG=409274

Review URL: https://codereview.chromium.org/504263002

Cr-Commit-Position: refs/heads/master@{#293205}