[ios] Explain compilation on ios-device.

Bug: 740301
Change-Id: I6a644c7a37af699dde3ac964143f42db3437ed3c
Reviewed-on: https://chromium-review.googlesource.com/571478Reviewed-by: smut <smut@google.com>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486602}

ios/build/bots/chromium.mac/ios-device.json

@@ -18,6 +18,7 @@
     "all"
   ],
   "configuration": "Release",
+  "explain": true,
   "sdk": "iphoneos10.0",
   "tests": [
   ]