Bug 1593319 - Set aFboId in DCLayerTree::Bind() r=gw,nical

EGLSurface could be bound only to framebuffer-zero.

Differential Revision: https://phabricator.services.mozilla.com/D51774

--HG--
extra : moz-landing-system : lando

gfx/webrender_bindings/DCLayerTree.cpp

@@ -183,6 +183,8 @@ void DCLayerTree::Bind(wr::NativeSurfaceId aId, wr::DeviceIntPoint* aOffset,
   MOZ_ASSERT(layer->GetEGLSurface() != EGL_NO_SURFACE);
   gl::GLContextEGL::Cast(mGL)->SetEGLSurfaceOverride(layer->GetEGLSurface());
   mCurrentId = Some(aId);
+
+  *aFboId = 0;
 }
 
 void DCLayerTree::Unbind() {