ppb_flash_menu: call gtk_menu_popup on browser thread
ppb_flash_menu: call gtk_menu_popup on browser thread

reuben/freshplayerplugin,grevutiu-gabriel/freshplayerplugin,Papapashu/freshplayerplugin,grevutiu-gabriel/freshplayerplugin,Papapashu/freshplayerplugin,Papapashu/freshplayerplugin,reuben/freshplayerplugin,reuben/freshplayerplugin