Don't access members in WebstoreInstaller::OnDownloadUpdated() after reporting failure.
Don't access members in WebstoreInstaller::OnDownloadUpdated() after reporting failure.

ReportFailure() releases a reference, which might destroy the object.

BUG=yup

Review URL: https://codereview.chromium.org/1525153002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#365513}

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