SkSize can be aggregate-initialized

SkSize can be aggregate-initialized

Previosly, SkSize had a base class, which prevented it.

Also removes unused SkISize::clampNegToZero() and
SkSize::clampNegToZero().

Change-Id: I7b93b42f6f6381c66e294bbedee99ad53c6c3436
Reviewed-on: https://skia-review.googlesource.com/13187
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>