It took a few months but we now know the U.S. Supreme Court's June 30 ruling in Harris v. Quinn will be implemented in Washington State. According to the tentative 2015-17 contract agreement between the state and SEIU 775 (Article 4):

Section 4.5 of the contract, however, provides an opportunity for the prior union security clause and forced unionization of individual providers to be enforced if a court rules that Harris v. Quinn doesn't apply to the union.

For now though the state will apparently implement the ruling from Harris v. Quinn pending the outcome of Centeno v. Dept. of Social & Health Services.

Additional Information
Does Harris v. Quinn apply to individual providers in Washington?
Attorney General declines Senator’s request for Harris v. Quinn impact opinion