[roll] Roll chromium to r503964 (#878)

This patch rolls chromium to r503964.

Note: since the plznavigate is not supported by puppeteer right now, the
patch also starts passing the `--disable-browser-side-navigation` flag.
This is a temporary work around for us.

References #877.

package.json

@@ -33,7 +33,7 @@
     "ws": "^3.0.0"
   },
   "puppeteer": {
-    "chromium_revision": "499413"
+    "chromium_revision": "503964"
   },
   "devDependencies": {
     "commonmark": "^0.27.0",