Specify iOS dimensions for WebRTC's tests like for Chromium's tests

Otherwise they're hitting an old Mac version and can't find xcode

Change-Id: Ie959d5fa958001822cace2160fb6c45516b8f602
Reviewed-on: https://chromium-review.googlesource.com/c/1355131Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Oleh Prypin <oprypin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612265}

ios/build/bots/chromium.webrtc.fyi/WebRTC Chromium FYI ios-simulator.json

@@ -22,17 +22,32 @@
     {
       "include": "webrtc_tests.json",
       "device type": "iPhone 6s Plus",
-      "os": "11.4"
+      "os": "11.4",
+      "dimensions": [
+        { "os": "Mac-10.13.4", "pool": "Chrome" },
+        { "os": "Mac-10.13.5", "pool": "Chrome" },
+        { "os": "Mac-10.13.6", "pool": "Chrome" }
+      ]
     },
     {
       "include": "webrtc_tests.json",
       "device type": "iPhone 6s",
-      "os": "11.4"
+      "os": "11.4",
+      "dimensions": [
+        { "os": "Mac-10.13.4", "pool": "Chrome" },
+        { "os": "Mac-10.13.5", "pool": "Chrome" },
+        { "os": "Mac-10.13.6", "pool": "Chrome" }
+      ]
     },
     {
       "include": "webrtc_tests.json",
       "device type": "iPad Air 2",
-      "os": "11.4"
+      "os": "11.4",
+      "dimensions": [
+        { "os": "Mac-10.13.4", "pool": "Chrome" },
+        { "os": "Mac-10.13.5", "pool": "Chrome" },
+        { "os": "Mac-10.13.6", "pool": "Chrome" }
+      ]
     }
   ]
 }