Add SkPath::isLastContourClosed()

Add SkPath::isLastContourClosed()

Adds a simple method for checking if the last command/verb in the
current contour is a 'close'.

This will simplify determining "closedness" for blink::Path, and aid
in the implementation of algorithms such as:

https://drafts.fxtf.org/motion-1/#motion-processing (second item in list)

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Review URL: https://codereview.chromium.org/1601103006