[build] Unify quotations in DEPS file

Bug: chromium:772804
Change-Id: I845a5ca4ff79c713eb6361ff99e4e09fe4c5c71d
Reviewed-on: https://chromium-review.googlesource.com/706792Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48382}