Fix variable shadowing in Viewer's ImGui code

This worked, but was confusing.

Bug: skia:
Change-Id: I9a387f2fc772277086e0f6ce532c834839b5e4dc
Reviewed-on: https://skia-review.googlesource.com/c/184800
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
1 file changed