Use Document::validBaseElementURL to init preloadscanner

Before this CL, Document::baseElementURL was used to initialize
PreloadScanner. However, this URL can be !isValid().

This CL introduces Document::validBaseElementURL() which returns
baseElementURL only when its valid, end returns empty URL when its invalid.
This can safely be used to initialize preload scanner.

BUG=623365

Review-Url: https://codereview.chromium.org/2132283002
Cr-Commit-Position: refs/heads/master@{#404595}
5 files changed