Allow creating a picture from skp to fail.

Allow creating a picture from skp to fail.

Replace the current constructor for creating an
SkPicturePlayback to a factory. In the factory,
check for incorrect data that would result in an invalid
playback. If the playback is invalid, return NULL, and
return NULL from SkPicture's factory as well.

Update SkTimedPicture(Playback) as well.

BUG=skia:1672
R=caryclark@google.com

Review URL: https://codereview.chromium.org/24826002

git-svn-id: http://skia.googlecode.com/svn/trunk@11554 2bbb7eff-a529-9590-31e7-b0007b416f81