zero-copy: Clarify to allocate/destroy GpuMemoryBuffer on any thread and use it on the main thread o

ui/gl/gl_image_surface_texture.cc

Index: ui/gl/gl_image_surface_texture.cc
diff --git a/ui/gl/gl_image_surface_texture.cc b/ui/gl/gl_image_surface_texture.cc
index a321c4d3092df158b49c350741ee2cd2048b8469..20e7b15634642e9d46d37e0178fe76d4f27cbee3 100644
--- a/ui/gl/gl_image_surface_texture.cc
+++ b/ui/gl/gl_image_surface_texture.cc
@@ -14,17 +14,20 @@ GLImageSurfaceTexture::GLImageSurfaceTexture(const gfx::Size& size)
 }
 
 GLImageSurfaceTexture::~GLImageSurfaceTexture() {
+  DCHECK(thread_checker_.CalledOnValidThread());
   DCHECK(!surface_texture_.get());
   DCHECK_EQ(0, texture_id_);
 }
 
 bool GLImageSurfaceTexture::Initialize(SurfaceTexture* surface_texture) {
+  DCHECK(thread_checker_.CalledOnValidThread());
   DCHECK(!surface_texture_.get());
   surface_texture_ = surface_texture;
   return true;
 }
 
 void GLImageSurfaceTexture::Destroy(bool have_context) {
+  DCHECK(thread_checker_.CalledOnValidThread());
   surface_texture_ = NULL;
   texture_id_ = 0;
 }
@@ -33,6 +36,7 @@ gfx::Size GLImageSurfaceTexture::GetSize() { return size_; }
 
 bool GLImageSurfaceTexture::BindTexImage(unsigned target) {
   TRACE_EVENT0("gpu", "GLImageSurfaceTexture::BindTexImage");
+  DCHECK(thread_checker_.CalledOnValidThread());
 
   if (target != GL_TEXTURE_EXTERNAL_OES) {
     LOG(ERROR)