Set temporary paths volatile so we don't cache them.

Set temporary paths volatile so we don't cache them.

Any path that is generated frame-to-frame should not be rendered by using the
DistanceFieldPathRenderer, because generating the initial distance field,
uploading it and rendering it takes longer than the SoftwarePathRenderer.

BUG=skia:2935

Review URL: https://codereview.chromium.org/677463002