Make ~SurfaceTextureGLOwner check for null Surface.

During GPU process teardown, the GpuCommandBufferStub clears the
current surface.  ScopedMakeCurrent doesn't handle this case well.

Since the process is going away, we just skip GL cleanup.

Bug: 718117
Change-Id: I04dce5d60fbe9d3f6c060c161029d7ec724ecdb0
Reviewed-on: https://chromium-review.googlesource.com/544381Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Chris Watkins <watk@chromium.org>
Commit-Queue: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#481739}

media/gpu/surface_texture_gl_owner.cc

@@ -70,6 +70,12 @@ SurfaceTextureGLOwnerImpl::~SurfaceTextureGLOwnerImpl() {
   // Make sure that the SurfaceTexture isn't using the GL objects.
   surface_texture_ = nullptr;
 
+  if (gl::GLSurface::GetCurrent() == nullptr) {
+    // This happens during GpuCommandBufferStub teardown.  Just leave -- the gpu
+    // process is going away.  crbug.com/718117 .
+    return;
+  }
+
   ui::ScopedMakeCurrent scoped_make_current(context_.get(), surface_.get());
   if (scoped_make_current.Succeeded()) {
     glDeleteTextures(1, &texture_id_);