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This declares a new GN arg save_unstripped_output that can be enabled if
enable_stripping=true. If true, then an unstripped copy of the linker output
will be saved in the output directory with an ".unstripped" suffix.

BUG=628052
R=mark@chromium.org

Review-Url: https://codereview.chromium.org/2157573002
Cr-Commit-Position: refs/heads/master@{#406110}