Reenable three iOS CQ builders.

Overall cycle time and compile step time have improved signficantly
on all three matching waterfall bots since the hardlink workaround
landed yesterday:
  - ios-device: http://shortn/_ow7Hr0gN6e
  - ios-device-xcode-clang: http://shortn/_qdlv49pNuS
  - ios-simulator-xcode-clang: http://shortn/_LlpTYE0H51

Bug: 740301, 739556
Change-Id: Iebf1b1398273d87580d5aaa295231f2a9e33bcaf
Reviewed-on: https://chromium-review.googlesource.com/579567Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: smut <smut@google.com>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488736}

infra/config/cq.cfg

@@ -67,15 +67,10 @@ verifiers {
     }
     buckets {
       name: "master.tryserver.chromium.mac"
-      # https://crbug.com/739556; make this non-experimental ASAP.
-      builders {
-        name: "ios-device"
-        experiment_percentage: 100
-      }
-      # https://crbug.com/739556
-      # builders { name: "ios-device-xcode-clang" }
+      builders { name: "ios-device" }
+      builders { name: "ios-device-xcode-clang" }
       builders { name: "ios-simulator" }
-      # builders { name: "ios-simulator-xcode-clang" }
+      builders { name: "ios-simulator-xcode-clang" }
       builders { name: "mac_chromium_compile_dbg_ng" }
       builders { name: "mac_chromium_rel_ng" }
     }