Other than the San Juan islands, there are no territorial disputes between the countries. The Canadian military is utterly dwarfed by the USA's, so any aggression on their part will likely result in a total loss. And if the USA really wanted Canadian territory, they could probably ask for it and be granted some in a popular vote (at least in the south western provinces).
Even if China and Canada become besties, China still can't do anything militarily. Canada is still gigantic; protecting the east of the country is impossible. And it would be straight forward for the USA to deny access to the Atlantic.
Yeah, the US government would not at all like the idea of China stationing troops or missiles in Canada, but Canada would see the stupidity of allowing such a move long before it came to pass. And if they didn't, the USA has diplomatic options (mainly, "hey, BC/Alberta/Sas, u wanna b american? :cowboy:"). Getting one to come to the red white and blue side effectively divides the country in half.