Reduce reflows in gr-main-header

Probing the documentation base URL would block the final layout of the
gr-main-header, and when this was slow, the header would reflow as the
page loads, causing minor jank.

Allow the docs link to probe in parallel.

Change-Id: Icb3ba0849251d0a608dcb6a696095b163b6dd679

polygerrit-ui/app/elements/core/gr-main-header/gr-main-header.js

@@ -111,6 +111,7 @@
       },
       _docBaseUrl: {
         type: String,
+        value: null,
       },
       _links: {
         type: Array,