Add GN configs for -fsanitizer=null.

build/config/sanitizers/BUILD.gn

Index: build/config/sanitizers/BUILD.gn
diff --git a/build/config/sanitizers/BUILD.gn b/build/config/sanitizers/BUILD.gn
index 7dd3dd2e62029fb06b28726329d0200ba498e3e8..979138865a2985d2c4e6d276ba20f027a4315775 100644
--- a/build/config/sanitizers/BUILD.gn
+++ b/build/config/sanitizers/BUILD.gn
@@ -127,6 +127,9 @@ config("default_sanitizer_ldflags") {
     if (is_ubsan || is_ubsan_security) {
       ldflags += [ "-fsanitize=undefined" ]

[Nico, 2016/06/02 21:06:09]

but is having to set a toggle really easier than just adding -fsanitize=null to
this line locally while working on this?

     }
+    if (is_ubsan_null) {
+      ldflags += [ "-fsanitize=null" ]
+    }
     if (is_ubsan_vptr) {
       ldflags += [ "-fsanitize=vptr" ]
     }
@@ -361,6 +364,12 @@ config("ubsan_security_flags") {
   }
 }
 
+config("ubsan_null_flags") {
+  if (is_ubsan_null) {
+    cflags = [ "-fsanitize=null" ]
+  }
+}
+
 config("ubsan_vptr_flags") {
   if (is_ubsan_vptr) {
     ubsan_vptr_blacklist_path =
@@ -383,6 +392,7 @@ all_sanitizer_configs = [
   ":tsan_flags",
   ":ubsan_flags",
   ":ubsan_no_recover",
+  ":ubsan_null_flags",
   ":ubsan_security_flags",
   ":ubsan_vptr_flags",
 ]