Previously, drafts were only based on line number and not patch range.
This adds range to the key in localstorage so that multiple drafts can
be stored for a line range. It also prevents the wrong draft from
surfacing in the incorrect context.
This change is also important for the multi-thread changes that are
currently in development as well, because it is possible to have
separate threads for these ranges, each of which can have its own draft
saved in storage.
Bug: Issue 3548
Change-Id: Id4c1beb5d73a47a1f98b0b169091905c80f8c64a
64ce59f48a