Remove String(RefPtr<StringImpl>) constructor

The String(RefPtr<StringImpl>) constructor should be removed for the following reasons:
1) It almost does not have clients, so it is unneeded.
2) RefPtr should never be passed by value

Review URL: https://chromiumcodereview.appspot.com/23872003

git-svn-id: svn://svn.chromium.org/blink/trunk@157321 bbb929c8-8fbe-4397-9dbb-9b2b20218538
DOMParser.parseFromString() should support creating HTML Document with mimetype text/html

The specification http://domparsing.spec.whatwg.org/#the-domparser-interface for DOMParser.parseFromString()
allows the document creation if the mimeType is one of the supported types: 
["text/html", "text/xml", "application/xml", "application/xhtml+xml", "image/svg+xml"]

Also script elements get marked unexecutable and the contents of noscript get parsed as markup.

R=tkent, abarth, eseidel

BUG=265379

Review URL: https://chromiumcodereview.appspot.com/23903014

git-svn-id: svn://svn.chromium.org/blink/trunk@157322 bbb929c8-8fbe-4397-9dbb-9b2b20218538