Ensure there is a valid ARC bridge service when performing an a11y action

It is possible that, before performing an action, the ARC++ container crashed.

Ensure that we don't also crash Chrome by dereferencing nullptr.

Bug: 902107
Change-Id: I09de402a41dfead09cee4685bc610a227740f9a5
Reviewed-on: https://chromium-review.googlesource.com/c/1318740
Commit-Queue: David Tseng <dtseng@chromium.org>
Reviewed-by: Yuki Awano <yawano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606117}

chrome/browser/chromeos/arc/accessibility/arc_accessibility_helper_bridge.cc

@@ -492,6 +492,12 @@ void ArcAccessibilityHelperBridge::OnAction(
 
   auto* instance = ARC_GET_INSTANCE_FOR_METHOD(
       arc_bridge_service_->accessibility_helper(), PerformAction);
+  if (!instance) {
+    // This case should probably destroy all trees.
+    OnActionResult(data, false);
+    return;
+  }
+
   instance->PerformAction(
       std::move(action_data),
       base::BindOnce(&ArcAccessibilityHelperBridge::OnActionResult,