Avoid setting current tab in DragHandle
Avoid setting current tab in DragHandle

This CL simplifies the data flow of currentTab: DragHandle does not
touch the global state variable at all anymore, instead relying on the
tab key passed by DetailsPanel that's guaranteed to be consistent with
the actual details panel currently shown.

Change-Id: I6ecb3b973d36a522ecdaa98440fd9716b3382b6e

google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto