Writing """\
additional_target_cpus = [ "x86" ]
goma_dir = "/b/build/slave/cache/goma_client"
is_component_build = false
is_debug = true
target_cpu = "x64"
target_os = "ios"
use_goma = true
""" to /b/build/slave/ios-simulator/build/src/out/Debug-iphonesimulator/args.gn.
/b/build/slave/ios-simulator/build/src/buildtools/mac/gn gen //out/Debug-iphonesimulator --check
  -> returned 1
ERROR at //v8/BUILD.gn:135:7: Duplicate build argument declaration.
      [ "//test/cctest/test-experimental-extra.js" ]
      ^--------------------------------------------
Here you're declaring an argument that was already declared elsewhere.
You can only declare each argument once in the entire build so there is one
canonical place for documentation and the default value. Either move this
argument to the build config file (for visibility everywhere) or to a .gni file
that you "import" from the files where you need it (preferred).
See //ios/BUILD.gn:15:41: Previous declaration.
  v8_experimental_extra_library_files = []
                                        ^
See also "gn help buildargs" for more on how build arguments work.
See //content/test/BUILD.gn:360:5: which caused the file to be included.
    "//v8",
    ^-----
GN gen failed: 1
step returned non-zero exit code: 1

Assign to Donn as it only affects one CL:
https://chromium-review.googlesource.com/c/chromium/src/+/977059

Leave it the sheriff queue just in case.

This problem is gone but there might be an underlying dependency bug that needs to be addressed somewhere in the ntp_snippets component. cc-ing ntp_snippets/OWNERS so they have a heads-up.

This issue came up during development of this CL.  https://chromium-review.googlesource.com/c/chromium/src/+/977059 
 I removed a dependency in the penultimate patchset which caused the problem to go away.  We got a few different kinds of errors, which all looked like a duplicate dependency that was not restricted to iOS.  

Huh, are you sure this is due to an issue in ntp_snippets and not just random flakiness (potentially in V8)? Nothing in the error message suggests ntp_snippets.

> are you sure this is due to an issue in ntp_snippets
No, but some of the flakes didn't have anything to do with V8 (see below), so just making a guess that it's in our component.

Also saw:
=============================
Writing """\
additional_target_cpus = [ "x86" ]
goma_dir = "/b/build/slave/cache/goma_client"
is_component_build = false
is_debug = true
target_cpu = "x64"
target_os = "ios"
use_goma = true
""" to /b/build/slave/ios-simulator/build/src/out/Debug-iphonesimulator/args.gn.
/b/build/slave/ios-simulator/build/src/buildtools/mac/gn gen //out/Debug-iphonesimulator --check
  -> returned 1
ERROR at //third_party/mesa/BUILD.gn:16:1: Assertion failed.
assert(!is_ios, "mesa is not used on iOS, don't pull it in accidentally")
^-----
mesa is not used on iOS, don't pull it in accidentally
See //cc/BUILD.gn:559:7: which caused the file to be included.
      "//third_party/mesa:osmesa",
      ^--------------------------
GN gen failed: 1
step returned non-zero exit code: 1
=============================