Fix Incognito Data Reduction Proxy nullptr crash.
Fix Incognito Data Reduction Proxy nullptr crash.

On Android, if safe browsing is disabled as a managed preference, when
loading pages in incognito, the browser will crash as DRPIOData is not
created, but is accessed when determining which resource throttles to
add.

BUG=480405

Review URL: https://codereview.chromium.org/1123323002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#328569}

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