Disable VS 2015 warning that fires on compiler-generated constructors.

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 d0b3329baf7d862c57509ca2ce059a4db861e3e5..fd586fd6ab44365866edcfb147ad97b57e5fcc0c 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -752,6 +752,12 @@ if (is_win) {
     # prevent it.
     "/wd4503",
 
+    # Warning C4589 says: "Constructor of abstract class ignores
+    # initializer for virtual base class." Disable this warning because it
+    # is flaky in VS 2015 RTM. It triggers on compiler generated
+    # copy-constructors in some cases.
+    "/wd4589",
+
     # C4611: interaction between 'function' and C++ object destruction is
     #        non-portable
     # This warning is unavoidable when using e.g. setjmp/longjmp.  MSDN