Fix meta-tag lookup for content detection in autosizing mode.

Lookup meta-tags in default namespace xhtmlNamespaceURI only
to avoid toHTMLMetaElement casting issues when the meta-tag
is in a different namespace.

BUG=291854

Review URL: https://chromiumcodereview.appspot.com/23621042

git-svn-id: svn://svn.chromium.org/blink/trunk@157979 bbb929c8-8fbe-4397-9dbb-9b2b20218538