Basic error checking for debugger GL widget.

Basic error checking for debugger GL widget.

Currently the debugger segfaults if the GL context cannot be created.
This change allows the debugger to build with GL support, but still run if the
GL context cannot be created at runtime.

R=robertphillips@google.com

Committed: https://code.google.com/p/skia/source/detail?r=9834

[bungeman-skia, 2013-06-29 20:51:27]

I was Chromoting over the weekend and found it rather annoying that debugger
would crash. I realize that the debugger can be built without GPU support, but
just making a few checks at runtime (which should probably be done anyway) lets
debugger run usefully instead of segfaulting when the GL context isn't available
at runtime.

[robertphillips, 2013-06-30 17:20:36]

lgtm

[bungeman-skia, 2013-07-01 13:54:11]

Committed patchset #1 manually as r9834 (presubmit successful).