[chromecast] Fix destruction in magnifier unit test.

Fails in memory sanitizer test downstream.

Bug: none
Test: unit test
Change-Id: If03106801056d192dd70eefd6d4ffaa443505391
Reviewed-on: https://chromium-review.googlesource.com/1114087Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Commit-Queue: Alex Sakhartchouk <alexst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570148}

chromecast/graphics/accessibility/partial_magnification_controller_unittest.cc

@@ -94,6 +94,14 @@ class PartialMagnificationControllerTest : public aura::test::AuraTestBase {
         std::make_unique<PartialMagnificationController>(root_window());
   }
 
+  void TearDown() override {
+    // PartialMagnificationController needs to be deleted before the root window
+    // is torn down by AuraTestBase.
+    controller_.reset();
+
+    aura::test::AuraTestBase::TearDown();
+  }
+
  protected:
   PartialMagnificationController* GetController() const {
     return controller_.get();