fix an instance of bugprone-unused-return-value

This new clang-tidy check fires when the result of a function like
`unique_ptr.release()` is unused. Sometimes this is intentional, other
times, it's not (e.g., the dev intended to `.reset()`).

Bug: 1134714
Change-Id: Ibb59ab44faba246b1d80f2abc0897238bf0c3e71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2457670Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Commit-Queue: George Burgess <gbiv@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825638}

content/renderer/accessibility/render_accessibility_impl.cc

@@ -292,7 +292,7 @@ void RenderAccessibilityImpl::DidCommitProvisionalLoad(
   }
   tree_source_->RemoveImageAnnotator();
   ax_image_annotator_->Destroy();
-  ax_image_annotator_.release();
+  ax_image_annotator_.reset();
   page_language_.clear();
 }
 
@@ -1285,7 +1285,7 @@ void RenderAccessibilityImpl::StartOrStopLabelingImages(ui::AXMode old_mode,
              !new_mode.has_mode(ui::AXMode::kLabelImages)) {
     tree_source_->RemoveImageAnnotator();
     ax_image_annotator_->Destroy();
-    ax_image_annotator_.release();
+    ax_image_annotator_.reset();
   }
 }