[PDF] Add helper class to manage ContentEntry set up and completion.

[PDF] Add helper class to manage ContentEntry set up and completion.

This stack object helper class calls finishContentEntry when it goes out of scope, maintains the current content entry, and manages the dst form xobject when it is needed.  This can be made cleaner by moving the guts of SkPDFDevice into a core object, which can expose setUp/finishContentEntry as public, but that is left as a todo.

Review URL: http://codereview.appspot.com/4515126

git-svn-id: http://skia.googlecode.com/svn/trunk@1409 2bbb7eff-a529-9590-31e7-b0007b416f81