Implement Dump-on-DCHECK (via a new LogSeverity).

build/config/BUILD.gn

Index: build/config/BUILD.gn
diff --git a/build/config/BUILD.gn b/build/config/BUILD.gn
index b9eb2ef50aae60463036b01f372b4d93ee90eaf9..39a9a9991c8d4fcad5fe85dba82ea5912e7102eb 100644
--- a/build/config/BUILD.gn
+++ b/build/config/BUILD.gn
@@ -58,9 +58,18 @@ declare_args() {
 config("feature_flags") {
   # Don't use deprecated V8 APIs anywhere.
   defines = [ "V8_DEPRECATION_WARNINGS" ]
+
   if (dcheck_always_on) {
     defines += [ "DCHECK_ALWAYS_ON=1" ]
   }
+
+  # This will cause DCHECKs to dump-without-crashing. See crbug.com/596231.
+  if (dump_on_first_dcheck) {
+    assert(is_debug || dcheck_always_on,
+           "dump_on_first_check requires is_debug, or dcheck_always_on")
+    defines += [ "DCHECK_IS_DUMP_WITHOUT_CRASH=1" ]
+  }
+
   if (use_udev) {
     # TODO(brettw) should probably be "=1".
     defines += [ "USE_UDEV" ]