Prevent infinite looping in ScriptContext::GetEffectiveDocumentURL().
Prevent infinite looping in ScriptContext::GetEffectiveDocumentURL().

BUG=568432

Review URL: https://codereview.chromium.org/1517633002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#364658}

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