Change chrome.dom namespace's channel to stable

This CL changes chrome.dom namespace's channel from dev to stable.

Bug: 778816
Change-Id: I4ed039f72330a14baee487e89da325ff1ca707e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532540
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826838}

chrome/common/extensions/api/_api_features.json

@@ -350,7 +350,7 @@
     "contexts": ["blessed_extension"]
   },
   "dom": {
-    "channel": "dev",
+    "channel": "stable",
     "contexts": ["blessed_extension", "content_script"]
   },
   "downloads": {