Fix that unable to find the NativePixmap error

When enable passthrough cmd decoder in CrOS, below errow are
printed repeatedly when accessing aquarium demo.
ERROR:overlay_processor_ozone.cc(212)] Unable to find the
NativePixmap corresponding to the overlay candidate

It seems that it will always return a nullptr when it's passthrough.
This change will remove this limitation.

Bug: chromium:1096442
Change-Id: I54faeedcc5ffe4bafae1f7a2a57ecd21c766ceb4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2311899
Commit-Queue: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: ccameron <ccameron@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791129}

gpu/command_buffer/service/shared_image_backing_gl_image.cc

@@ -358,9 +358,6 @@ GLuint SharedImageBackingGLImage::GetGLServiceId() const {
 }
 
 scoped_refptr<gfx::NativePixmap> SharedImageBackingGLImage::GetNativePixmap() {
-  if (IsPassthrough())
-    return nullptr;
-
   return image_->GetNativePixmap();
 }