Update how SkPath handles fGenerationID and fSourcePath, and add tests to cover.

include/core/SkPath.h

Index: include/core/SkPath.h
diff --git a/include/core/SkPath.h b/include/core/SkPath.h
index be15a63c8901686724b390703e86b8a3728c4faa..cc4dfcb494f2c654f58c0ee7c86d54fa42d6100c 100644
--- a/include/core/SkPath.h
+++ b/include/core/SkPath.h
@@ -166,12 +166,14 @@ public:
 
     /** Clear any lines and curves from the path, making it empty. This frees up
         internal storage associated with those segments.
+        On Android, does not change fSourcePath.
     */
     void reset();
 
     /** Similar to reset(), in that all lines and curves are removed from the
         path. However, any internal storage for those lines/curves is retained,
         making reuse of the path potentially faster.
+        On Android, does not change fSourcePath.
     */
     void rewind();