Migrating ios13-beta-sim Goma RBE in MB

ios13-beta-sim has moved to Chromium and utilizes mb_config.pyl.
Adding ios_use_goma_rbe=true there.

Change-Id: I504bb005e77d7e1b4818e54179a1c46aee3389c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2098331Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Jeff Yoon <jeffyoon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749271}

tools/mb/mb_config.pyl

@@ -2346,10 +2346,14 @@
     },
 
     'ios_simulator': {
-      'mixins': ['ios'],
+      'mixins': ['ios', 'ios_use_goma_rbe' ],
       'gn_args': 'target_cpu="x64"',
     },
 
+    'ios_use_goma_rbe': {
+      'gn_args': 'ios_use_goma_rbe=true'
+    },
+
     'libfuzzer': { 'gn_args': 'use_libfuzzer=true' },
 
     'link_jobs_32': {