[ios] -Wno-null-pointer-arithmetic is Xcode 10 only.

crrev.com/c/1090151 moved the null-pointer-arithmetic warning too far back.
It's supported by Xcode 10, but not Xcode 9.3.

Bug: 850333, 843234
Change-Id: I21935c592ca7b3ba75cb6fc7b7c0686564dd3ec4
Reviewed-on: https://chromium-review.googlesource.com/1099649Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567046}

build/config/compiler/BUILD.gn

@@ -1445,7 +1445,11 @@ config("default_warnings") {
 
           # TODO(hans): https://crbug.com/681136
           "-Wno-unused-lambda-capture",
-
+        ]
+      }
+      if (current_toolchain == host_toolchain || !use_xcode_clang ||
+          xcode_version_int >= 1000) {
+        cflags += [
           # TODO(hans): https://crbug.com/766891
           "-Wno-null-pointer-arithmetic",
         ]