Reset focus before re-using TSF document.

Due to the fact that we reuse TSF document for text fields with same
input type, we should property reset the state by clearing focus before
re-using the document.

Bug: 1162974
Change-Id: Ia92187705e93cc064e992e28c8638d5647e06ea9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2610147Reviewed-by: Yohei Yukawa <yukawa@chromium.org>
Reviewed-by: Siye Liu <siliu@microsoft.com>
Commit-Queue: Siye Liu <siliu@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#840322}

ui/base/ime/win/tsf_bridge.cc

@@ -233,6 +233,13 @@ void TSFBridgeImpl::OnTextInputTypeChanged(const TextInputClient* client) {
     return;
   }
 
+  // Since we reuse TSF document for same text input type, there is a case where
+  // focus is switched between two text fields with same input type. We should
+  // prepare the TSF document for reuse by clearing focus first.
+  if (input_type_ != TEXT_INPUT_TYPE_NONE &&
+      input_type_ == client_->GetTextInputType()) {
+    thread_manager_->SetFocus(nullptr);
+  }
   input_type_ = client_->GetTextInputType();
   TSFDocument* document = GetAssociatedDocument();
   if (!document)