[Linux] Do not try to call readdir() on a null DIR* in base::ProcessIterator.

base/process/process_iterator_linux.cc

Index: base/process/process_iterator_linux.cc
diff --git a/base/process/process_iterator_linux.cc b/base/process/process_iterator_linux.cc
index a9d044c53824f7cd74f1a1c9717a51f0fefc5dfb..95275c29abff7c966c24810df4bfd1bc3b28322a 100644
--- a/base/process/process_iterator_linux.cc
+++ b/base/process/process_iterator_linux.cc
@@ -59,18 +59,28 @@ bool GetProcCmdline(pid_t pid, std::vector<std::string>* proc_cmd_line_args) {
 ProcessIterator::ProcessIterator(const ProcessFilter* filter)
     : filter_(filter) {
   procfs_dir_ = opendir(internal::kProcDir);
+  if (!procfs_dir_) {
+    // On Android, SELinux may prevent reading /proc. See
+    // https://crbug.com/581517 for details.
+    PLOG(ERROR) << "opendir " << internal::kProcDir;
+  }
 }
 
 ProcessIterator::~ProcessIterator() {
   if (procfs_dir_) {
     closedir(procfs_dir_);
-    procfs_dir_ = NULL;
+    procfs_dir_ = nullptr;
   }
 }
 
 bool ProcessIterator::CheckForNextProcess() {
   // TODO(port): skip processes owned by different UID
 
+  if (!procfs_dir_) {
+    DLOG(ERROR) << "Skipping CheckForNextProcess(), no procfs_dir_";
+    return false;
+  }
+
   pid_t pid = kNullProcessId;
   std::vector<std::string> cmd_line_args;
   std::string stats_data;