[PDF] Implement clear xfermode with a mask instead of pushing the clip down.

This approach is much less likely to use an SkRegion and ends up working better for Webkit.

Review URL: http://codereview.appspot.com/4550057

git-svn-id: http://skia.googlecode.com/svn/trunk@1375 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/include/pdf/SkPDFDevice.h b/include/pdf/SkPDFDevice.h
index 94c30d8..0f634aa 100644
--- a/include/pdf/SkPDFDevice.h
+++ b/include/pdf/SkPDFDevice.h
@@ -166,6 +166,10 @@
     void cleanUp();
     void createFormXObjectFromDevice(SkRefPtr<SkPDFFormXObject>* xobject);
 
+    // Clear the passed clip from all existing content entries.
+    void clearClipFromContent(const SkClipStack* clipStack,
+                              const SkRegion& clipRegion);
+
     // If the paint or clip is such that we shouldn't draw anything, these
     // return false and do not create a content entry.
     bool setUpContentEntry(const SkClipStack* clipStack,