Use Document::validBaseElementURL to init preloadscanner

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}
(cherry picked from commit 51b5da563d004465a203dd40892a13b8543721e1)

[kouhei (in TOK), 2016-07-18 02:43:07]

Committed patchset #1 (id:1) to pending queue manually as
d54432fbea838c2c3976b7d353526d0298d8f36d.