[ios] Set symbol_level=0 for ios-device* builds

Both ios-device and ios-device-xcode-clang seem to have disk space issues. This
is an attempt to reduce the disk space required for compilation on bots.

Bug: 895761
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: Iea34c089b780ecb9231ab2fc9031ae9f89ab998f
Reviewed-on: https://chromium-review.googlesource.com/c/1285770Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Sergey Berezin <sergeyberezin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600433}

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

@@ -10,6 +10,7 @@
     "ios_enable_code_signing=false",
     "is_component_build=false",
     "is_debug=false",
+    "symbol_level=0",
     "target_cpu=\"arm\"",
     "target_os=\"ios\"",
     "use_goma=true",

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

@@ -11,6 +11,7 @@
     "ios_enable_code_signing=false",
     "is_component_build=false",
     "is_debug=false",
+    "symbol_level=0",
     "target_cpu=\"arm\"",
     "target_os=\"ios\"",
     "use_goma=true"