Various steps are failing on https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/ios-webview with "XcodeVersionNotFoundError: Xcode version not found: %s".

Failing builds:
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/ios-webview/17
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/ios-webview/16
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/ios-webview/15
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/ios-webview/14
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/ios-webview/13

Example log:
[P20293 21:11:08.073 reader.go:380 I] cipd: extracting - 100%
[P20293 21:12:13.090 client.go:1594 I] All changes applied.
dyld: Symbol not found: __os_log_disabled
  Referenced from: /b/s/w/ir/Xcode.app/Contents/SharedFrameworks/DVTInstrumentsFoundation.framework/Versions/A/../../../DVTInstrumentsUtilities.framework/Versions/A/DVTInstrumentsUtilities (which was built for Mac OS X 10.13)
  Expected in: /usr/lib/libSystem.B.dylib
 in /b/s/w/ir/Xcode.app/Contents/SharedFrameworks/DVTInstrumentsFoundation.framework/Versions/A/../../../DVTInstrumentsUtilities.framework/Versions/A/DVTInstrumentsUtilities
dyld: Symbol not found: __os_log_disabled
  Referenced from: /b/s/w/ir/Xcode.app/Contents/SharedFrameworks/DVTInstrumentsFoundation.framework/Versions/A/../../../DVTInstrumentsUtilities.framework/Versions/A/DVTInstrumentsUtilities (which was built for Mac OS X 10.13)
  Expected in: /usr/lib/libSystem.B.dylib
 in /b/s/w/ir/Xcode.app/Contents/SharedFrameworks/DVTInstrumentsFoundation.framework/Versions/A/../../../DVTInstrumentsUtilities.framework/Versions/A/DVTInstrumentsUtilities
[E 2018-10-23 21:12:20] original error: signal: abort trap
[E 2018-10-23 21:12:20] 
[E 2018-10-23 21:12:20] goroutine 1:
[E 2018-10-23 21:12:20] #0 mac_toolchain/install.go:191 - main.finalizeInstall.func1()
[E 2018-10-23 21:12:20]   reason: failed to install Xcode packages
[E 2018-10-23 21:12:20] 
[E 2018-10-23 21:12:20] #1 mac_toolchain/install.go:115 - main.RunWithXcodeSelect()
[E 2018-10-23 21:12:20] #2 mac_toolchain/install.go:188 - main.finalizeInstall()
[E 2018-10-23 21:12:20] #3 mac_toolchain/install.go:237 - main.installXcode()
[E 2018-10-23 21:12:20] #4 mac_toolchain/main.go:115 - main.(*installRun).Run()
[E 2018-10-23 21:12:20] #5 github.com/maruel/subcommands/subcommands.go:360 - subcommands.Run()
[E 2018-10-23 21:12:20] #6 mac_toolchain/main.go:239 - main.main()
[E 2018-10-23 21:12:20] #7 runtime/proc.go:198 - runtime.main()
[E 2018-10-23 21:12:20] #8 runtime/asm_amd64.s:2361 - runtime.goexit()
Xcode build version 10a254a failed to install: Command '['./mac_toolchain', 'install', '-kind', 'ios', '-xcode-version', '10a254a', '-output-dir', 'Xcode.app']' returned non-zero exit status 1
Traceback (most recent call last):
  File "src/ios/build/bots/scripts/run.py", line 73, in main
    xctest=args.xctest,
  File "/b/s/w/ir/src/ios/build/bots/scripts/test_runner.py", line 679, in __init__
    xctest=xctest,
  File "/b/s/w/ir/src/ios/build/bots/scripts/test_runner.py", line 342, in __init__
    raise XcodeVersionNotFoundError(xcode_build_version)
XcodeVersionNotFoundError: ('Xcode version not found: %s', '10a254a')