[sheriff] Add scoped task environment to failing a11y tests.

The error message is saying to add a scoped task environment.

TBR=nektar@chromium.org

Bug: crbug.com/909062
Change-Id: I5aca973574fd8754bdfc1ef44af1319a494b3d13
Reviewed-on: https://chromium-review.googlesource.com/c/1352869
Commit-Queue: Darren Shen <shend@chromium.org>
Reviewed-by: Darren Shen <shend@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611521}

content/browser/accessibility/browser_accessibility_unittest.cc

@@ -4,6 +4,7 @@
 
 #include "content/browser/accessibility/browser_accessibility.h"
 
+#include "base/test/scoped_task_environment.h"
 #include "build/build_config.h"
 #include "content/browser/accessibility/browser_accessibility_manager.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ -16,6 +17,7 @@ class BrowserAccessibilityTest : public testing::Test {
   ~BrowserAccessibilityTest() override;
 
  private:
+  base::test::ScopedTaskEnvironment task_environment_;
   DISALLOW_COPY_AND_ASSIGN(BrowserAccessibilityTest);
 };