Tiny fix in Chromevox smart sticky mode test set up

Change-Id: Ie6b5eadc25d2ddda95140d6c2730e663a33e34c3
AX-Relnotes: n/a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2439699
Commit-Queue: Josiah Krutz <josiahk@google.com>
Auto-Submit: Josiah Krutz <josiahk@google.com>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812431}

chrome/browser/resources/chromeos/accessibility/chromevox/background/smart_sticky_mode_test.js

@@ -14,7 +14,7 @@ ChromeVoxSmartStickyModeTest = class extends ChromeVoxNextE2ETest {
     this.ssm_ = new SmartStickyMode();
     // Deregister from actual range changes.
     ChromeVoxState.removeObserver(this.ssm_);
-    assertFalse(ssm.didTurnOffStickyMode_);
+    assertFalse(this.ssm_.didTurnOffStickyMode_);
   }
 
   assertDidTurnOffForNode(node) {