Fix DCHECK() crash in ArcImeService.

It should use WMHelper::HasInstance() to check there is WMHelper
instance, but WMHelper::GetInstance() was used instead.

Bug: 911857
Change-Id: I46249edd757eff5364863365983be6bef905505c
Reviewed-on: https://chromium-review.googlesource.com/c/1365093Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Commit-Queue: Yuichiro Hanada <yhanada@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614285}

components/arc/ime/arc_ime_service.cc

@@ -54,7 +54,7 @@ class ArcWindowDelegateImpl : public ArcImeService::ArcWindowDelegate {
   ~ArcWindowDelegateImpl() override = default;
 
   bool IsInArcAppWindow(const aura::Window* window) const override {
-    if (!exo::WMHelper::GetInstance())
+    if (!exo::WMHelper::HasInstance())
       return false;
     aura::Window* active = exo::WMHelper::GetInstance()->GetActiveWindow();
     for (; window; window = window->parent()) {