[ios-simulator] Use symbol_level=1 for debug builds

This should be enough for printing traces, and will save some precious disk
space on the bots.

R=justincohen@chromium.org

Bug: 834527
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I0ff1210c48aeba8abb4639eba6a334559085fda7
Reviewed-on: https://chromium-review.googlesource.com/1018497Reviewed-by: smut <smut@google.com>
Commit-Queue: Sergey Berezin <sergeyberezin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552881}

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

@@ -10,6 +10,7 @@
     "goma_dir=\"$(goma_dir)\"",
     "is_component_build=false",
     "is_debug=true",
+    "symbol_level=1",
     "target_cpu=\"x64\"",
     "target_os=\"ios\"",
     "use_goma=true"