Add --resetGpuContext to both DM and nanobench.

dm/DMTask.cpp

Index: dm/DMTask.cpp
diff --git a/dm/DMTask.cpp b/dm/DMTask.cpp
index 54e7df3803a763324992b65b0c808ca2447d56f5..b7635563094e34279897020133db8be98b736005 100644
--- a/dm/DMTask.cpp
+++ b/dm/DMTask.cpp
@@ -4,6 +4,7 @@
 
 DEFINE_bool(cpu, true, "Master switch for running CPU-bound work.");
 DEFINE_bool(gpu, true, "Master switch for running GPU-bound work.");
+DEFINE_bool(resetGpuContext, true, "Reset the GrContext before running each task.");
 
 DECLARE_bool(dryRun);
 
@@ -70,6 +71,9 @@ GpuTask::GpuTask(Reporter* reporter, TaskRunner* taskRunner) : Task(reporter, ta
 
 void GpuTask::run(GrContextFactory& factory) {
     if (FLAGS_gpu && !this->shouldSkip()) {
+        if (FLAGS_resetGpuContext) {
+            factory.destroyContexts();
+        }
         this->start();
         if (!FLAGS_dryRun) this->draw(&factory);
         this->finish();