Navigation: Replace hardcoded kAboutBlankURL checks with IsAboutBlank().

This is to use the more reliable GURL::IsAboutBlank() when checking for
blank pages instead of just matching the urls.

Change-Id: Ieb5692cf1da8e323baa2e72285d2801d2fc0d1ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1583786
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Mohamed Abdelhalim <zetamoo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654066}