An analytical model is developed to evaluate the performance of an IEEE 802.11ac Wireless Local Area Network (WLAN) in support of delay sensitive video services over multiple channels. Specifically, the channel bonding probability and the channel access delay of wireless users are analyzed, considering the contentions among legacy and ac users in the same channel and across multiple channels. Based on the analysis, the network capacity region, i.e., the maximum number of traffic flows that can be supported with the bounded delay performance in a multi-channel WLAN with and without channel bonding, is derived. Our analysis shows that channel bonding can improve the network capacity when the channel is under-utilized with a small number of legacy users co-existing with the ac users; yet channel bonding is not always favorable and it may degrade the network capacity when the number of legacy users increases due to the increased contentions in the network. The analysis provides important guidance for effective admission control and channel bonding strategies to guarantee the bonded service delay of realtime applications. Extensive simulations validate the analysis.