We use a 'destSlide' entry in the local storage to keep presentation
and notes in sync while using '-notes' flag. This caused a name
collision if we had different presentations open at the same time.
This changes the 'destSlide' key in local storage to
'destSlide:<presentation-url>' to avoid collisions between different
presentations.
Fixesgolang/go#24688
Change-Id: I4833fa83856a96b5978c09ed7e4360484ba4f109
Reviewed-on: https://go-review.googlesource.com/c/tools/+/178659
Reviewed-by: Andrew Bonventre <andybons@golang.org>