Add m_isGlyphLookupMode to UniscribeHelper class and set it to true when filling up glyph pages for non-BMP
codepoints. When it's set to true, UniscribeHelper does not do its own font fallback. Neither does it
glyph placement. Font fallback will be taken care of in the simple script font path. For simple script, glyph
placement is not necessary.
(see also http://crbug.com/6304