cast: make CastTouchEventGateTest remove pre-target handler

CastTouchEventGateTest adds a pre-target handler, but it never removes it.
I'm going to add some DCHECKs to ensure there are no pre-target handlers when
an object is destroyed, so this needs to be fixed.

BUG=867035
TEST=none

Change-Id: I2b3804ba9a96288d209958af7c8703e99732d32b
Reviewed-on: https://chromium-review.googlesource.com/c/1281044Reviewed-by: Sergey Volk <servolk@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599667}

chromecast/graphics/cast_touch_event_gate_test.cc

@@ -51,6 +51,7 @@ class CastTouchEventGateTest : public aura::test::AuraTestBase {
   }
 
   void TearDown() override {
+    root_window()->RemovePreTargetHandler(event_handler_.get());
     event_gate_.reset();
     aura::test::AuraTestBase::TearDown();
   }