[Autocorrect] add IsAvailable check before clearing

Bug: 1117498
Change-Id: I09c348442df6a23143c1bbdadbf1f5ecbb1add0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2362239
Commit-Queue: My Nguyen <myy@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799463}

third_party/blink/renderer/core/editing/ime/input_method_controller.cc

@@ -1779,6 +1779,11 @@ WebTextInputType InputMethodController::TextInputType() const {
 
 void InputMethodController::WillChangeFocus() {
   FinishComposingText(kKeepSelection);
+
+  // Event handler might destroy document.
+  if (!IsAvailable())
+    return;
+
   GetDocument().Markers().RemoveSuggestionMarkerByType(
       SuggestionMarker::SuggestionType::kAutocorrect);
 }