cc: Remove use of PassAs() and constructor-casting with scoped_ptr.

cc/output/software_renderer.cc

Index: cc/output/software_renderer.cc
diff --git a/cc/output/software_renderer.cc b/cc/output/software_renderer.cc
index 538bb6c204e5d93537715db0d9f96c3a70bb267c..ae58b63304a10172b73165c9f2849c5f2ad53833 100644
--- a/cc/output/software_renderer.cc
+++ b/cc/output/software_renderer.cc
@@ -107,7 +107,7 @@ void SoftwareRenderer::BeginDrawingFrame(DrawingFrame* frame) {
 
 void SoftwareRenderer::FinishDrawingFrame(DrawingFrame* frame) {
   TRACE_EVENT0("cc", "SoftwareRenderer::FinishDrawingFrame");
-  current_framebuffer_lock_.reset();
+  current_framebuffer_lock_ = nullptr;
   current_canvas_ = NULL;
   root_canvas_ = NULL;
 
@@ -150,7 +150,7 @@ void SoftwareRenderer::Finish() {}
 
 void SoftwareRenderer::BindFramebufferToOutputSurface(DrawingFrame* frame) {
   DCHECK(!output_surface_->HasExternalStencilTest());
-  current_framebuffer_lock_.reset();
+  current_framebuffer_lock_ = nullptr;
   current_canvas_ = root_canvas_;
 }
 
@@ -158,7 +158,6 @@ bool SoftwareRenderer::BindFramebufferToTexture(
     DrawingFrame* frame,
     const ScopedResource* texture,
     const gfx::Rect& target_rect) {
-  current_framebuffer_lock_.reset();
   current_framebuffer_lock_ = make_scoped_ptr(
       new ResourceProvider::ScopedWriteLockSoftware(
           resource_provider_, texture->id()));