There is an important distinction between temporarily leaving the United States and leaving permanently, which is at least all that is dictated by your return to your place of residence.
If you are in the United States under the Visa Waiver Program (VWP) and you leave the United States for a few weeks to visit Canada (or Mexico) for the purpose of to return to the United States, you are deemed to have left only temporarily. this period, and therefore your 90 days, even if you are out of the country. At the time of the I-94W forms (the green form that you completed when you arrived in the United States), you kept your I-94W in your passport, even when you left the United States.
If you leave the United States to return to your place of residence, you leave permanently. In this case, your VWP period would end when you left the country and, historically, you would have returned your I-94W.
Since you have a work visa to be in Canada, Canada would be your place of residence for the period of this visa. This means that every time you leave the United States, your visit will be deemed to be completed upon your return to your place of residence and will definitely leave the United States – which means that when you return to the United States, United, the VWP clock will be restarted.
In the end, the situation between Canada and Mexico is intended to prevent people from submitting to US visa requirements in Canada / Mexico, which gives them 90 more days on their VWP. If it is clear that it is not what you do, you will have no problem.
Do not forget that to enter the United States under the VWP, you must first get an ESTA.
(Note: the usual provisions apply.) IANAL, but I have entered the US about 40 times on VWP and on Visas, so I know the process pretty well! If you want a definitive answer, contact us. USCIS or contact a qualified immigration officer (lawyer)