Set ios_use_goma_rbe=true in the latest builder.

To experiment ios_use_goma_rbe config behavior, let me enable this.

Bug: 1044515
Change-Id: I2870eb540186f89b03b2faae07265ebcf5e090bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2014480
Auto-Submit: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Jason Clinton <jclinton@chromium.org>
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734347}

ios/build/bots/chromium.fyi/ios-device-goma-rbe-latest-clobber.json

 {
   "comments": [
     "Goma RBE latest builder for iOS.",
-    "It is chromium.mac/ios-device.json + use_goma_canary, clobber."
+    "It is chromium.mac/ios-device.json + use_goma_latest, clobber."
   ],
   "xcode build version": "11a1027",
   "gn_args": [
@@ -12,7 +12,8 @@
     "symbol_level=0",
     "target_cpu=\"arm64\"",
     "target_os=\"ios\"",
-    "use_goma=true"
+    "use_goma=true",
+    "ios_use_goma_rbe=true"
   ],
   "additional_compile_targets": [
     "all"