SYDNEY (Reuters) - Actor Mel Gibson and his Russian girlfriend Oksana Grigorieva are celebrating the birth of a daughter, according to media reports. Gibson, 53, announced in May that his girlfriend was pregnant. Celebrity website RadarOnline.com said 39-year-old Grigorieva gave birth on Friday, a few weeks early. There were no further details. The Australian-raised actor already has seven children from his first wife Robyn who filed for divorce in April this year after 28 years of marriage. Grigorieva has a teenage son with former James Bond actor Timothy Dalton. Gibson has said that he and his wife split up three years ago, shortly after he was arrested in Malibu for drunken driving and launched into an alcohol-fueled tirade against...