Set error reason properly for not-implemented WebURLLoaderImpl::loadSynchronously
Set error reason properly for not-implemented WebURLLoaderImpl::loadSynchronously

So that the caller doesn't treat this load as a success which triggers assertion later.

BUG=513789

Review URL: https://codereview.chromium.org/1266503007

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#341377}

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