DM: --gpu and --cpu should only control top-level tasks.

This fixes a bug where we run some Android bots with --nocpu, and the
current behavior disables the (CPU-bound) WriteTasks the GPU bound GM
runs spawn off.  The WriteTasks don't run and we end up with "null" in
our .json files.

Tested locally: out/Release/dm --nocpu -w /tmp/out; ls /tmp/out
  dm.json  gpu/

BUG=skia:2938
R=jcgregorio@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/578033002
diff --git a/dm/DMTask.cpp b/dm/DMTask.cpp
index 5ceb53b..3fa5c39 100644
--- a/dm/DMTask.cpp
+++ b/dm/DMTask.cpp
@@ -46,7 +46,9 @@
 CpuTask::CpuTask(const Task& parent) : Task(parent) {}
 
 void CpuTask::run() {
-    if (FLAGS_cpu && !this->shouldSkip()) {
+    // If the task says skip, or if we're starting a top-level CPU task and we don't want to, skip.
+    const bool skip = this->shouldSkip() || (this->depth() == 0 && !FLAGS_cpu);
+    if (!skip) {
         this->start();
         if (!FLAGS_dryRun) this->draw();
         this->finish();
@@ -64,7 +66,9 @@
 GpuTask::GpuTask(Reporter* reporter, TaskRunner* taskRunner) : Task(reporter, taskRunner) {}
 
 void GpuTask::run(GrContextFactory* factory) {
-    if (FLAGS_gpu && !this->shouldSkip()) {
+    // If the task says skip, or if we're starting a top-level GPU task and we don't want to, skip.
+    const bool skip = this->shouldSkip() || (this->depth() == 0 && !FLAGS_gpu);
+    if (!skip) {
         this->start();
         if (!FLAGS_dryRun) this->draw(factory);
         this->finish();