Use upper case hex in viewer's zoom window

Bug: skia:
Change-Id: I5452dd557e103ccf75fb70312d32ad7615a830ed
Reviewed-on: https://skia-review.googlesource.com/c/193468
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>