Clang: Disable new -Wnull-pointer-arithmetic warning

TBR=rnk

Bug: 766891
Change-Id: I1275ee0d199955d89645248903fdd9451be7f36f
Reviewed-on: https://chromium-review.googlesource.com/674038
Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#502999}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: a6421c589c9c93d9719787866763c84be80f24e2

config/compiler/BUILD.gn

@@ -1242,6 +1242,12 @@ config("default_warnings") {
         # TODO(hans): https://crbug.com/763392
         "-Wno-tautological-unsigned-zero-compare",
       ]
+      if (llvm_force_head_revision) {
+        cflags += [
+          # TODO(hans): https://crbug.com/766891
+          "-Wno-null-pointer-arithmetic",
+        ]
+      }
     } else if (use_xcode_clang && xcode_version_int >= 830) {
       # This is necessary to allow a progressive transition from using xcode 8.0
       # to 8.3 or more recent. Remove when all bots are migrated to 8.3.