Reland of Suppress -Wnonportable-include-path on clang tot bots (patchset #1 id:1 of https://codereview.chromium.org/2043813002/ )

Reason for revert:
Warning landed again upstream.

Original issue's description:
> Revert of Suppress -Wnonportable-include-path on clang tot bots (patchset #2 id:20001 of https://codereview.chromium.org/2040533003/ )
>
> Reason for revert:
> The Clang warning was removed again in r271761.
>
> Original issue's description:
> > Suppress -Wnonportable-include-path on clang tot bots
> >
> > BUG=617318
> > R=hans@chromium.org
> >
> > Committed: 2bc751024e
>
> TBR=thakis@chromium.org
> # Not skipping CQ checks because original CL landed more than 1 days ago.
> BUG=617318
>
> Committed: https://crrev.com/2b359d2e5b4f7639055c9c6ff3ed39a03a2a3a0b
> Cr-Commit-Position: refs/heads/master@{#398065}

TBR=hans@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=617318

Review-Url: https://codereview.chromium.org/2069603002
Cr-Original-Commit-Position: refs/heads/master@{#399714}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 7d9add3bd7c9655d92641fb2aac427d1cb417095

common.gypi

@@ -3037,6 +3037,10 @@
         # TODO: Enable on Windows too, http://crbug.com/404525
         'variables': { 'clang_warning_flags': ['-Wexit-time-destructors']},
       }],
+      ['"<!(python <(DEPTH)/tools/clang/scripts/update.py --print-revision)"!="270823-1"', {
+        # TODO(thakis): https://crbug.com/617318
+        'variables': { 'clang_warning_flags': ['-Wno-nonportable-include-path']},
+      }],
       ['chromium_code==0', {
         'variables': {
           'clang_warning_flags': [

config/compiler/BUILD.gn

@@ -1014,6 +1014,13 @@ config("default_warnings") {
         # TODO(thakis): https://crbug.com/604888
         "-Wno-undefined-var-template",
       ]
+
+      if (llvm_force_head_revision) {
+        cflags += [
+          # TODO(thakis): https://crbug.com/617318
+          "-Wno-nonportable-include-path",
+        ]
+      }
     }
   }
 }