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

build/config/dcheck_always_on.gni

Index: build/config/dcheck_always_on.gni
diff --git a/build/config/dcheck_always_on.gni b/build/config/dcheck_always_on.gni
index 60672cdf50e6d22e5bff5b04d972976b084e3f05..d31f69431b951c281a1f9b3ebb94548b7ffa7076 100644
--- a/build/config/dcheck_always_on.gni
+++ b/build/config/dcheck_always_on.gni
@@ -4,5 +4,12 @@
 
 declare_args() {
   # Set to true to enable dcheck in Release builds.
-  dcheck_always_on = false
+  # TODO(596231): Default to enabled on Windows Official builds.
+  dcheck_always_on = is_official_build && is_win
+
+  # Set to true to have the first failed dcheck cause a dump to be
+  # uploaded, without crashing. Implies dcheck_always_on, but does not
+  # actually enable logging of dcheck failures.
+  # TODO(596231): Default to enabled on Windows Official builds.
+  dump_on_first_dcheck = is_official_build && is_win
 }