Fix bound returned by SkPath::isRect when the path contains a trailing moveTo

Oddly enough this was fixed in:

https://codereview.chromium.org/16950021/ (add rect-output parameter to isRect, allowing us to return the correct bounds even if a rectagular path has a trailing moveTo)

but was reverted here:

https://skia.googlesource.com/skia/+/8fd160350ca5f57fbb1b2e03383c5778414a9b48

since it appeared to be crashing Chrome's trybots. I think it just fell through the cracks after that.

If this sticks I will land a follow on patch for the stroke issue reported in the original bug (crbug.com/247770).

BUG=247770,445368

Review URL: https://codereview.chromium.org/834483002