This contains the following commits:

flutter/engine@5822b6e Clear the canvas when attempting to render to the surface.
flutter/engine@d8eb659 Make the rasterizer own the compositor context.
flutter/engine@387945c Correct ninja command instruction
flutter/engine@00911b2 Don't send platform messages to isolates that are not running.
flutter/engine@5c857ee Set the start timestamp in the contructor of the first shell.