Add flag-metadata for dcheck-is-fatal.

Mark dcheck-is-fatal as a special never-expiring flag, since it is used
with special DCHECK-enabled builds to explicitly opt-in (or out) of
crashing when a check fails.

Bug: 901515
Change-Id: I153082352eaaa048db9d4070f802b4ce1ab8c38a
Reviewed-on: https://chromium-review.googlesource.com/c/1351732Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611464}

chrome/browser/flag-metadata.json

@@ -463,8 +463,8 @@
   },
   {
     "name": "dcheck-is-fatal",
-    // "owners": [ "your-team" ],
-    "expiry_milestone": 76
+    "owners": [ "wez" ],
+    "expiry_milestone": -1
   },
   {
     "name": "debug-packed-apps",