Steps to reproduce:
1. Open user attached project (BugReport1)
2. Make a WebGL build
3. Run the game on browser
4. Try to move scroll bar using mouse click
5. Observe the "MissingMethodException: Method not found" exception in the console

Expected result: OnGUI slider scrolling should be properly working with no exceptions
Actual result: OnGUI slicer scrolling does not work when clicked and dragged, exception is thrown

Reproduced in: 2017.3.0a2, 2017.2.0b5, 2017.2.0b1
Not reproduced in: 2017.2.0a4, 2017.1.0p2, 5.6.3f1, 5.5.4p2
Regression since: 2017.2.0b1