Fix setShadowLayer + drawTextOnPath

Bug: 140255441
Test: I49c468b0f4cc142e5d6b03f0aee917b3d85153f3

Fix two issues that prevented this combination from working:
- make() the SkTextBlob once and reuse it for each iteration of the
  loop. This is necessary because make() resets the SkTextBlobBuilder,
  so a second call does nothing.
- use the altered SkPaint, passed as a parameter to the lambda, rather
  than the Paint passed to apply_looper.
Incidentally, reference mCanvas directly like the other methods, rather
than calling asSkCanvas(), which returns mCanvas.

Change-Id: I48ea1232b12df4f5f8afffc20c7dc8e5dc2bb511