The isolate driver finds dlls automatically. Hardcoding them
blocks gn as the locations change.

Prepares for https://codereview.chromium.org/2303233002/

BUG=chromium:474921
NOTRY=true
TBR=jochen,vogelheim

Review-Url: https://codereview.chromium.org/2304723003
Cr-Commit-Position: refs/heads/master@{#39125}