Fix NULL ptr with short-lived TextureImageTransportSurfaces

When the surface is created from GpuCommandBufferStub, it is
made current for the very first time while
helper_->stub()->decoder()->GetGLContext() returns NULL (see order
in GpuCommandBufferStub::OnInitialize()).

If we then handle drawing and swapping of a frame while
a) the context is still current and we avoid calling
Surface::OnMakeCurrent() again
b) the surface is destroyed right after with the ack pending,
it is possible for context_ to be NULL in BufferPresentedImpl.

With virtual contexts, this patch causes us to hold a reference
to the real GL context instead now, which is fine because we only
need the context to delete the backbuffer texture during destruction.

BUG=169429

Review URL: https://codereview.chromium.org/12252028

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@182403 0039d316-1c4b-4281-b951-d872f2087c98

content/common/gpu/texture_image_transport_surface.cc

@@ -114,10 +114,7 @@ bool TextureImageTransportSurface::OnMakeCurrent(gfx::GLContext* context) {
     return true;
   }
 
-  if (!context_.get()) {
-    DCHECK(helper_->stub());
-    context_ = helper_->stub()->decoder()->GetGLContext();
-  }
+  context_ = context;
 
   if (!fbo_id_) {
     glGenFramebuffersEXT(1, &fbo_id_);