Reset the cached QuickAnswers result.

Quick Answer result is cached when getting it from the server. The
result type is then used for logging purpose.

Currently, the result doesn't get reset when user select a new text. If
users perform actions before the result came back from the server, the
cached result type is used for logging.

This CL reset the result when user select a new text and trigger
QuickAnswers.

Bug: b/160244136
Test: Manual test
Change-Id: I526684755b7876aa0b6df12513ead9284a5454c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2276513
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Li Lin <llin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784116}

ash/quick_answers/quick_answers_controller_impl.cc

@@ -77,6 +77,7 @@ void QuickAnswersControllerImpl::MaybeShowQuickAnswers(
   title_ = title;
   query_ = title;
   context_ = context;
+  quick_answer_.reset();
 
   QuickAnswersRequest request = BuildRequest();
   if (chromeos::features::IsQuickAnswersTextAnnotatorEnabled()) {