On which border is the US a military and economic threat to China? If you're suggesting the US has a military presence in Pakistan that threatens China, then my response would have to be gut busting laughter. If you're suggesting the American presence in Afghanistan is a threat to China...well, let's just say I'm eagerly awaiting your thesis.

It seems to me China's foreign policy is geared toward non-confrontation. China seems much more likely to seek an alternate route for Iranian oil to flow through Pakistan than engage in hostilities in the Gulf. Too much to lose, too little to gain. China has plenty of cash to invest in energy infrastructure. Running a pipeline through Pakistan would be far less expensive in both economic and geopolitical terms than backing a globally disruptive play like closing the Straits, US aggression or no. China's position, from what I've read over the years, has been geared toward multipolar diplomacy and away from the use of its military. They've got hundreds of millions of people to lift out of abject poverty, and as such are more concerned with supply chains and commodity prices than going to war with it's largest trading partner.

EDIT: Bonus points for describing how China would project force in the Persian Gulf.