Add an override to use C++11

This is used for projects that rely on Chromium's build infrastructure
after Chromium switched to C++14
https://chromium-review.googlesource.com/c/583635

Bug: 750683
Change-Id: Id673a2474a1b5c606ce17b1ce8e7faa4de0cb5b9
Reviewed-on: https://chromium-review.googlesource.com/596870Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Oleh Prypin <oprypin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491195}

build/config/compiler/BUILD.gn

@@ -104,6 +104,9 @@ declare_args() {
   # on by default when goma is enabled because setting this to true may make
   # it harder to debug binaries.
   strip_absolute_paths_from_debug_symbols = false
+
+  # Allow projects that wish to stay on C++11 to override Chromium's default.
+  use_cxx11 = false
 }
 
 if (is_clang && !is_nacl) {
@@ -457,14 +460,23 @@ config("compiler") {
     # TODO(thakis): Eventually switch this to c++11 instead,
     # http://crbug.com/427584
     cflags_c += [ "-std=gnu11" ]
-    cflags_cc += [ "-std=gnu++14" ]
+    if (use_cxx11) {
+      # Override Chromium's default for projects that wish to stay on C++11.
+      cflags_cc += [ "-std=gnu++11" ]
+    } else {
+      cflags_cc += [ "-std=gnu++14" ]
+    }
   } else if (!is_win && !is_nacl) {
     # TODO(mcgrathr) - the NaCl GCC toolchain doesn't support either gnu11/gnu++11
     # or c11/c++11; we technically don't need this toolchain any more, but there
     # are still a few buildbots using it, so until those are turned off
     # we need the !is_nacl clause and the (is_nacl && is_clang) clause, above.
     cflags_c += [ "-std=c11" ]
-    cflags_cc += [ "-std=c++14" ]
+    if (use_cxx11) {
+      cflags_cc += [ "-std=c++11" ]
+    } else {
+      cflags_cc += [ "-std=c++14" ]
+    }
   }
 
   if (is_mac) {