Fix "reorder" warning about Scroll member.

The vtkWebInteractionEvent constructor was initializing its members
in a different order than the one in which they had been declared.
This caused a warning with gcc's "-Wreorder" option.

Change-Id: Iac50713f0fabd8610b9c5cc47a92fa170a917bdb
Merge topic 'web-reorder-warning' into master

8e0a9776 Fix "reorder" warning about Scroll member.