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Apart from the places which expect a SyncLoadResult, it's only read in
RedirectToFileResourceHandler. In that case, it's always net::OK;
ResourceLoader checks request_->status().is_success() first in
OnResponseStarted before calling CompleteResponseStarted().

BUG=none

Review URL: https://codereview.chromium.org/517853004

Cr-Commit-Position: refs/heads/master@{#293216}