Regarding the US, that's a bit of a misleading statement. US Census Bureau projections have "white" people as a majority in the US until 2060, which is as far out as their projections go [1].
The claim that "whites" are going to become a minority soon is arrived at by redefining "white" to exclude white-identified Hispanic people. Indeed, "non-Hispanic whites" are projected to become a minority in the US by 2045. But "whites" overall are not going to become a minority in the foreseeable future.
In 2060, the US Census Bureau projects the US will be 68% white-identified (white-identified Hispanic people included). By comparison, their actual estimates for 2016 are that the US was 76% white-identified (again, Hispanic people included). Although their projections don't go out that far, it seems likely the US will still have a white-identified majority at the start of the next century.
[1] https://www2.census.gov/programs-surveys/popproj/tables/2017...