Add --resetGpuContext to both DM and nanobench.

dm/DMTask.cpp

 #include "DMTask.h"
 #include "DMTaskRunner.h"
 #include "SkCommandLineFlags.h"
 
 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);
 
 namespace DM {
 
 Task::Task(Reporter* reporter, TaskRunner* taskRunner)
     : fReporter(reporter)
     , fTaskRunner(taskRunner)
     , fDepth(0) {
     fReporter->taskCreated();
@@ skipping 46 matching lines @@
     // Run children serially on this (CPU) thread.  This tends to save RAM and is usually no slower.
     // Calling spawnChildNext() is nearly equivalent, but it'd pointlessly contend on the
     // threadpool; spawnChildNext() is most useful when you want to change threadpools.
     task->run();
 }
 
 GpuTask::GpuTask(Reporter* reporter, TaskRunner* taskRunner) : Task(reporter, taskRunner) {}
 
 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();
     }
     SkDELETE(this);
 }
 
 void GpuTask::spawnChild(CpuTask* task) {
     // Really spawn a new task so it runs on the CPU threadpool instead of the GPU one we're on now.
     // It goes on the front of the queue to minimize the time we must hold reference bitmaps in RAM.
     this->spawnChildNext(task);
 }
 
 }  // namespace DM