Move fontmgr_bounds label setting to checkbox.

The fontmgr_bounds gm has an option to show the name of the font and the
glyph ids of the glyphs with extreme bounds. This was previously done
with a command line flag label_fontmgr_bounds. However, this information
is generally only useful in the viewer application, so change this to a
control.

This also adds bool field handling in viewer.

Change-Id: Idbdbb08e9516ec49a96f9baa320acd6fe44e95ca
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/203175
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
2 files changed