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/DMTask.cpp b/dm/DMTask.cpp
index d0a82b6..05eda4e 100644
--- a/dm/DMTask.cpp
+++ b/dm/DMTask.cpp
@@ -68,7 +68,10 @@
         this->start();
         if (!FLAGS_dryRun) this->draw(&factory);
         this->finish();
-        if (FLAGS_resetGpuContext) {
+        if (FLAGS_abandonGpuContext) {
+            factory.abandonContexts();
+        }
+        if (FLAGS_resetGpuContext || FLAGS_abandonGpuContext) {
             factory.destroyContexts();
         }
     }