Fix "unused variable" warnings.

cc/trees/single_thread_proxy.h

Index: cc/trees/single_thread_proxy.h
diff --git a/cc/trees/single_thread_proxy.h b/cc/trees/single_thread_proxy.h
index c513964f7742454c22950222098c19cc69e7b76c..b5e6691a9bb8cb5194ff9ebe59a47d523ec50b31 100644
--- a/cc/trees/single_thread_proxy.h
+++ b/cc/trees/single_thread_proxy.h
@@ -175,13 +175,16 @@ class CC_EXPORT SingleThreadProxy : public Proxy,
 // code is running on the impl thread to satisfy assertion checks.
 class DebugScopedSetImplThread {
  public:
+#if DCHECK_IS_ON()
   explicit DebugScopedSetImplThread(TaskRunnerProvider* task_runner_provider)
       : task_runner_provider_(task_runner_provider) {
-#if DCHECK_IS_ON()
     previous_value_ = task_runner_provider_->impl_thread_is_overridden_;
     task_runner_provider_->SetCurrentThreadIsImplThread(true);
-#endif
   }
+#else
+  explicit DebugScopedSetImplThread(TaskRunnerProvider* task_runner_provider) {}
+#endif
+
   ~DebugScopedSetImplThread() {
 #if DCHECK_IS_ON()
     task_runner_provider_->SetCurrentThreadIsImplThread(previous_value_);
@@ -189,8 +192,10 @@ class DebugScopedSetImplThread {
   }
 
  private:
+#if DCHECK_IS_ON()
   bool previous_value_;
   TaskRunnerProvider* task_runner_provider_;
+#endif
 
   DISALLOW_COPY_AND_ASSIGN(DebugScopedSetImplThread);
 };
@@ -199,13 +204,16 @@ class DebugScopedSetImplThread {
 // code is running on the main thread to satisfy assertion checks.
 class DebugScopedSetMainThread {
  public:
+#if DCHECK_IS_ON()
   explicit DebugScopedSetMainThread(TaskRunnerProvider* task_runner_provider)
       : task_runner_provider_(task_runner_provider) {
-#if DCHECK_IS_ON()
     previous_value_ = task_runner_provider_->impl_thread_is_overridden_;
     task_runner_provider_->SetCurrentThreadIsImplThread(false);
-#endif
   }
+#else
+  explicit DebugScopedSetMainThread(TaskRunnerProvider* task_runner_provider) {}
+#endif
+
   ~DebugScopedSetMainThread() {
 #if DCHECK_IS_ON()
     task_runner_provider_->SetCurrentThreadIsImplThread(previous_value_);
@@ -213,8 +221,10 @@ class DebugScopedSetMainThread {
   }
 
  private:
+#if DCHECK_IS_ON()
   bool previous_value_;
   TaskRunnerProvider* task_runner_provider_;
+#endif
 
   DISALLOW_COPY_AND_ASSIGN(DebugScopedSetMainThread);
 };