Call notifyPixelsChanged after modifying bitmap

Call notifyPixelsChanged after modifying bitmap

In CodecSrc's scanline_subset test, we decode a subset of an image to a
bitmap, draw it to the canvas, and then repeat. This is fine for most
backends, but not for pipe. Pipe sees the same generation ID, so it
assumes it is the same bitmap it saw before, and just draws the
original one.

Call notifyPixelsChanged, so the bitmap will get a new generation ID,
fixing pipe.

BUG=skia:4138

Review URL: https://codereview.chromium.org/1265983004