DM: add --quiet

dm/DMReporter.cpp

 #include "DMReporter.h"
 
+#include "SkCommandLineFlags.h"
+
+DEFINE_bool(quiet, false, "If true, don't print status updates.");
+
 namespace DM {
 
 void Reporter::updateStatusLine() const {
+    if (FLAGS_quiet) {
+        return;
+    }
+
     SkString status;
     status.printf("\r\033[K%d / %d", this->finished(), this->started());
     const int failed = this->failed();
     if (failed > 0) {
         status.appendf(", %d failed", failed);
     }
     SkDebugf(status.c_str());
 }
 
 int32_t Reporter::failed() const {
     SkAutoMutexAcquire reader(&fMutex);
     return fFailures.count();
 }
 
 void Reporter::fail(SkString name) {
     SkAutoMutexAcquire writer(&fMutex);
     fFailures.push_back(name);
 }
 
 void Reporter::getFailures(SkTArray<SkString>* failures) const {
     SkAutoMutexAcquire reader(&fMutex);
     *failures = fFailures;
 }
 
 }  // namespace DM