Fix double lock of texture resource in software renderer

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 ceedac658d717d74862979a26ce152c8dad4a99e..48ac91ad4e37778a445ae099c2882613e40e5206 100644
--- a/cc/output/software_renderer.cc
+++ b/cc/output/software_renderer.cc
@@ -160,6 +160,7 @@ bool SoftwareRenderer::BindFramebufferToTexture(
     DrawingFrame* frame,
     const ScopedResource* texture,
     const gfx::Rect& target_rect) {
+  current_framebuffer_lock_ = nullptr;
   current_framebuffer_lock_ = make_scoped_ptr(
       new ResourceProvider::ScopedWriteLockSoftware(
           resource_provider_, texture->id()));