Remove bogus ability for creating an SkPicturePlayback to fail.

Remove bogus ability for creating an SkPicturePlayback to fail.

Change SkPicturePlayback::parseBufferTag to return void, since
it can never return false.

Change SkPicturePlayback::parseStreamTag to return void, since
the only way it can return false is if parseBufferTag returns
false, or if creating a sub picture failed, both of which are
nonsensical.

Due to the above, there is no reason for creating an
SkPicturePlayback to fail, so remove the isValid parameter.

Update subclasses in SkDebuggerGUI.

Committed: https://code.google.com/p/skia/source/detail?r=7844

[reed1, 2013-02-25 13:41:25]

lgtm

[robertphillips, 2013-02-25 13:57:37]

lgtm