O(1) SkPictureUtils::ApproxBytesUsed()

Chrome wants to call this more often, and it's quite slow today.

Seems like this could be clearer if SkPictureUtils::ApproxBytesUsed() were SkPicture::approxBytesUsed().

BUG=chromium:471873

Review URL: https://codereview.chromium.org/1090943004
9 files changed