Copy dbghelp.dll to output directory

Chrome's tests sometimes symbolize call stacks. In order to support new
PDB formats such as PDB's generated by VS 2017's updated /debug:fastlink
we need to use a recent version of dbghelp.dll. This change restores a
modified version of _CopyDebugger that was removed a few weeks ago, thus
avoiding a CPU busy hang in the system version of dbghelp.dll.

Cr-Commit-Position: refs/heads/master@{#465784}
1 file changed