If the user starts writing a diff comment, but discards it or navigates
away before saving it as a draft, then the text that had been entered
re-appears if the user starts a comment on the same line of the same
file of the same patch-set of the same change.
Achieves this by storing the comment text in localStorage along with a
timestamp whenever the textarea is edited by the user. The entry is
cleared from localStorage if the user saves the comment as a draft. When
a new comment is started, the gr-diff-comment checks localStorage to see
whether a relevant entry exists to use as the initial text.
Adds the gr-storage element as an interface for localStorage. This
element clears away stored comment drafts if they are more than a day
old.
Bug: Issue 3787
Change-Id: I11327a69d463a6a84a0cd8d59f4662a6a4c296a6
7a4aa8cc81