This introduce DiffViewFullyLoaded and changing DiffViewDisplayed to
same convetion as ChangeViewDisplayed and ChangeViewFullyLoaded. This
also introduce DiffViewContentDisplayed which is event when content
displayed but we dont wait for a highlighting layer.
DiffViewDisplayed ends once we render 120 lines in diff, which is more
than full window viewport. DiffViewFullyLoaded ends when every line is
loaded same as before DiffViewDisplayed.
Change-Id: I99a0d44950054316305d4166e89b4e4f12eeb7a9
432c29ecb6