[ios] Enable goma on Xcode10b4 xcode_clang FYI bot.

Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I60c506d9c15bce437c32c844162ec5df243bb501
Reviewed-on: https://chromium-review.googlesource.com/1142957
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576483}

ios/build/bots/chromium.fyi/ios12-sdk-xcode-clang.json

 {
   "comments": [
-    "Tests for super-fat iOS 12.0.",
-    "Build is performed with gn+ninja."
+    "Tests for iOS 12.0 beta xcode clang. Goma can be enabled for beta 4 and",
+    "the final GM, but disabled for the other betas to reduce swirl for the",
+    "goma team. Build is performed with gn+ninja."
   ],
   "xcode build version": "10l213o",
   "use xcode build version": true,
@@ -13,7 +14,8 @@
     "symbol_level=1",
     "target_cpu=\"x64\"",
     "target_os=\"ios\"",
-    "use_goma=false",
+    "use_goma=true",
+    "goma_dir=\"$(goma_dir)\"",
     "use_xcode_clang=true"
   ],
   "env": {