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Currently if LOG_TO_FILE is set but log_file_path is null it'll crash
without warning. This CL adds a DCHECK to make this requirement clear.

Bug: 1137080
Change-Id: Icc7747e7d0ed2d698d13ddc58751ba6ccf97d5c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463074
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815963}