Reland of Disable "taking address of packed member" clang warning.

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 30d801789d8a36aeab008f75aeedea2afacebe79..bc23d304ecad2082a831633fab47b0aeda626df4 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -1022,6 +1022,13 @@ config("default_warnings") {
         # TODO(thakis): https://crbug.com/617318
         "-Wno-nonportable-include-path",
       ]
+
+      if (llvm_force_head_revision) {
+        cflags += [
+          # TODO(eugenis): Clean up, http://crbug.com/619640
+          "-Wno-address-of-packed-member",
+        ]
+      }
     }
   }
 }