How to reproduce:
1. Open attached project "ViewToolRepro.zip" and scene "ReproScene"
2. In the Scene view, hold right click and move through the scene (FPS/Flythrough mode)
3. In the Scene view, hold Alt+Left Click and move through the scene (Orbit mode)
4. In the Scene view, zoom in/out

Expected result: Tools.viewTool returns corresponding enum value (None, Orbit, Pan, Zoom, FPS)
Actual result: Tools.viewTool always returns Pan

Reproducible with: 2017.4.29f1, 2018.4.3f1, 2019.1.9f1, 2019.2.0b8, 2019.3.0a8