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Current Qtwebkit configuration is debug build by default.
Change it to release build to have better performance.

Change-Id: Ib5e10bfd62edf79ddc304332ff10fdbe3abf58f0
Reviewed-by: Chenxi Mao <chenxi.mao2013@gmail.com>
Reviewed-by: Konstantin Tokarev <annulen@yandex.ru>