Frame Selected centers around the last valid stroke.
When Symmetry is enabled the last mirrored `location` was added to the
`average_stroke_accum` in stead of the original stroke location.
This patch will add the `true_location` to the `average_stroke_accum`.
This contains the original stroke location.
Issue happened in Vertex and Weight paint.
Reviewed By:
Pablo Dobarro
Differential Revision: https://developer.blender.org/D6161
5840c17970