Test abandoning GL context in dm/nanobench.

Rename GrContext::contextDestroyed to GrContext::abandonContext.

Remove GrContext::resetContext.

R=robertphillips@google.com

Author: bsalomon@google.com

Review URL: https://codereview.chromium.org/422903002
diff --git a/dm/DMGpuSupport.h b/dm/DMGpuSupport.h
index 209f322..a3c487d 100644
--- a/dm/DMGpuSupport.h
+++ b/dm/DMGpuSupport.h
@@ -48,6 +48,8 @@
                                kNative_GLContextType = 0,
                                kNull_GLContextType   = 0;
     void destroyContexts() {}
+
+    void abandonContexts() {}
 };
 
 namespace DM {