Skia provides SkTime which is a platform-independent way to get at the system time.
As far as a "frame", that's whatever you want to consider it to be. The usual approach is to measure the time between swaps of the backbuffer (e.g., that's swapBuffers() in sk_app::WindowContext). To do that you could start a timer when you start drawing content, and then stop and record it when you start drawing content for the next frame (and start a new one, etc.). But you could measure the time from when you start drawing the content to when you call SkCanvas::flush(). That would leave out any time taken up by the swapchain, however, so may not be what you're looking for.
But other than the SkTime interface this is outside of Skia really -- StackOverflow has some examples of how to do this in general.