Reland "[ios] Move xcode-clang builders to Xcode 10."

This is a reland of eb25f32f

The CL was reverted with
https://chromium-review.googlesource.com/c/chromium/src/+/1282949
but it looks like an infra problem instead

TBR=justincohen@chromium.org,jbudorick@chromium.org
Original change's description:
> [ios] Move xcode-clang builders to Xcode 10.
>
> Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
> Change-Id: Ie5bdc140b82f49151ca891b39f65112bf6210eac
> Reviewed-on: https://chromium-review.googlesource.com/c/1278332
> Reviewed-by: Justin Cohen <justincohen@chromium.org>
> Reviewed-by: John Budorick <jbudorick@chromium.org>
> Commit-Queue: Justin Cohen <justincohen@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#599804}

Change-Id: I335dff58404b7bab44a5eab3c5b0337b38a572e3
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Reviewed-on: https://chromium-review.googlesource.com/c/1283016
Commit-Queue: Christos Froussios <cfroussios@chromium.org>
Reviewed-by: Christos Froussios <cfroussios@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599957}

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

@@ -3,7 +3,7 @@
     "Builder for 32-bit devices.",
     "Build is performed with gn+ninja."
   ],
-  "xcode build version": "10l213o",
+  "xcode build version": "10a254a",
   "gn_args": [
     "additional_target_cpus=[ \"arm64\" ]",
     "goma_dir=\"$(goma_dir)\"",

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

@@ -4,7 +4,7 @@
     "This 'xcode build version' must exist in GOMA",
     "Build is performed with gn+ninja."
   ],
-  "xcode build version": "10l213o",
+  "xcode build version": "10a254a",
   "use xcode build version": true,
   "gn_args": [
     "additional_target_cpus=[\"x86\"]",