Highlight reached/unreached lines differently in Debugger.

This is useful to know because breakpoints can only meaningfully be set
on a line of code which is reachable. In other words, we know the
entire execution path ahead of time, so we can tell you that some
branches aren't actually taken without needing to set a breakpoint at
all.

Change-Id: I72b2cddcf92fd6f75ea12d302230bae42b7112bc
Bug: skia:12666
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/485218
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
4 files changed