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Currently, when distillation fails, the TaskTracker signals that a
source has finished. It then checks if all sources are done (and if not,
it will wait to call the callback). The check for sources done though
assumes that if distiller_ is non-null, then distillation is still in
progress. So, we should be deleting/releasing distiller_ when it
finished (and before signalling that the source is finished).

This also updates the standalone content extractor to fail if extraction
of some url fails.

Review URL: https://codereview.chromium.org/575883002

Cr-Commit-Position: refs/heads/master@{#295599}