One-liner fix that allows Viewer to switch between Raster and OpenGL

One-liner fix that allows Viewer to switch between Raster and OpenGL

A previous change to Viewer removed a key setTag() call that
meant that the logic to replace the SurfaceView on a toggle
between OpenGL/Raster was not able to execute, making the app
useless after that toggle was performed.

BUG=skia:

Change-Id: Idddb7612209ce83d3461155c59f451407118e1d5
Reviewed-on: https://skia-review.googlesource.com/8789
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Chet Haase <chet@google.com>