Small optimization to ResourceLoader's canExpandStylesheetWith()

Small optimization to ResourceLoader's canExpandStylesheetWith()

The import CSS at-rule can only go in the very top of a style sheet.
(Technically, the charset at-rule may precede it, but that rule is not valid
for style sheets that are embedded in <style> tags.) So we don't need to scan
the entire CSS text for '@import' -- we just have to make sure that the CSS
text doesn't start with it.

Change-Id: I56c2ebd55d4a7e90dde8e89e76b6d1b52ed004f8