viz: Fix params->background_texture == 0 case

In crrev.com/c/1296679 we removed unnecessary code given the assumption that
backdrop_texture could never be 0.

Unfortunately we missed the case where background_rect is empty, and we still
can have params->background_texture equal to 0.

This patch brings back that check.

Bug: 908293, 904981
Change-Id: I94a1109c839337b77589c69fa607d7dc074c0daf
Reviewed-on: https://chromium-review.googlesource.com/c/1351711Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611417}

components/viz/service/display/gl_renderer.cc

@@ -1062,8 +1062,6 @@ void GLRenderer::UpdateRPDQShadersForBlending(
           DCHECK(params->background_image_id || IsContextLost());
         }
       }
-    }
-
       if (params->background_image_id) {
         // Reset original background texture if there is not any mask.
         if (!quad->mask_resource_id()) {
@@ -1072,11 +1070,16 @@ void GLRenderer::UpdateRPDQShadersForBlending(
         }
       } else if (CanApplyBlendModeUsingBlendFunc(blend_mode) &&
                  ShouldApplyBackgroundFilters(quad, params->backdrop_filters)) {
-      // Something went wrong with applying background filters to the backdrop.
+        // Something went wrong with applying background filters to the
+        // backdrop.
         params->use_shaders_for_blending = false;
         gl_->DeleteTextures(1, &params->background_texture);
         params->background_texture = 0;
       }
+    } else {  // params->background_rect.IsEmpty()
+      DCHECK(!params->background_image_id);
+      params->use_shaders_for_blending = false;
+    }
   }
 
   // Need original background texture for mask?