Don't preload scripts with invalid type/language attributes

This patch keeps track of a script tag's type and language attributes,
and gates preloading if they are invalid. This aligns the preload scanner's
policy with the ScriptLoader's policy.

BUG=623109

Review-Url: https://codereview.chromium.org/2099853002
Cr-Commit-Position: refs/heads/master@{#403182}