Fix remaining incompatibilities between scoped_ptr and unique_ptr.

scoped_ptr is more convertible to bool than std::unique_ptr.
  bool x = scoped_ptr<int>();  // compiles
  bool y = std::unique_ptr<int>();  // explodes

std::unique_ptr is not streamable:
  LOG(ERROR) << scoped_ptr<int>();  // compiles
  LOG(ERROR) << std::unique_ptr<int>();  // explodes

BUG=579269,579270
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1609923002

Cr-Commit-Position: refs/heads/master@{#370266}
diff --git a/cc/trees/single_thread_proxy.cc b/cc/trees/single_thread_proxy.cc
index 6e73dfa..5898c7a 100644
--- a/cc/trees/single_thread_proxy.cc
+++ b/cc/trees/single_thread_proxy.cc
@@ -98,7 +98,7 @@
 
 bool SingleThreadProxy::IsStarted() const {
   DCHECK(task_runner_provider_->IsMainThread());
-  return layer_tree_host_impl_;
+  return !!layer_tree_host_impl_;
 }
 
 bool SingleThreadProxy::CommitToActiveTree() const {