Fix tooltip on network error pages in iframes.
Fix tooltip on network error pages in iframes.

For network errors with no details, it was displaying "null".
Now there won't be a tooltip for errors without details.

BUG=543132

Review URL: https://codereview.chromium.org/1403703007

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#355021}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk