[PDF] Fix setting of existing clip for layers.

[PDF] Fix setting of existing clip for layers.

The current approach of setting the existing clip just before drawing a layer into a device doesn't work.  SkDevice::clear() uses existing clip before that and if we need to put the content in a transparency group (i.e. for SrcIn xfermode), we need a valid existing clip.  Instead, change the factory to use a special constructor when creating a layer device.

Review URL: http://codereview.appspot.com/4495041

git-svn-id: http://skia.googlecode.com/svn/trunk@1270 2bbb7eff-a529-9590-31e7-b0007b416f81