Add toString methods to SkImageFilter-derived classes

This isn't definitive but at least makes something show up in the debugger.

Review URL: https://codereview.chromium.org/789163006
mega rebaseline --tiny differences

BUG=skia:
TBR=
NOTRY=True

Review URL: https://codereview.chromium.org/802433004
Rebaseline (and ignore failure) on flaky NexusPlayer MPD GMs

BUG=skia:3267
TBR=reed@google.com

Review URL: https://codereview.chromium.org/814983004
fix leak

NOTREECHECKS=True
TBR=
BUG=skia:

Review URL: https://codereview.chromium.org/817083002