Flip 'Google Chrome Mac' builder on chromium.chrome back to GYP.

Looks like dump_syms is broken ...

TBR=rsesek@chromium.org, mark@chromium.org
NOTRY=true
BUG=622406

Review-Url: https://codereview.chromium.org/2083383004
Cr-Commit-Position: refs/heads/master@{#401362}