gn/win: Copy dbghelp.dll to the output dir.
gn/win: Copy dbghelp.dll to the output dir.

This was done for gyp in https://codereview.chromium.org/951083003/.
It was missing in gn, but base.isolate depends on this file. This makes
"isolate tests" fail on Windows bots that use gn and swarming.

BUG=460506,461160,498033

Review URL: https://codereview.chromium.org/1683793003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#374672}

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