[Spellchecker] Clear RequestTextCheckCallback safely in case Java object initialization fails

Mojo doesn't allow removing an uncalled mojo callback while the message pipe is
still open.

On Android, SpellCheckerSessionBridge::RequestTextCheck() discards an incoming
request if it fails to initialize the Java object. In this case, this patch replies
the request with empty result so Mojo requirements are ensured.

Bug: 848483
Change-Id: Ic4e1528dd65d152ad522aa3eb49c76f90a4e66b9
Reviewed-on: https://chromium-review.googlesource.com/1093962Reviewed-by: Rachel Blum <groby@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567813}

components/spellcheck/browser/spellchecker_session_bridge_android.cc

@@ -38,6 +38,12 @@ void SpellCheckerSessionBridge::RequestTextCheck(
     const base::string16& text,
     RequestTextCheckCallback callback) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
+
+  // This allows us to discard |callback| safely in case it's not run due to
+  // failures in initialization of |java_object_|.
+  std::unique_ptr<SpellingRequest> incoming_request =
+      std::make_unique<SpellingRequest>(text, std::move(callback));
+
   // SpellCheckerSessionBridge#create() will return null if spell checker
   // service is unavailable.
   if (java_object_initialization_failed_) {
@@ -71,11 +77,11 @@ void SpellCheckerSessionBridge::RequestTextCheck(
   // If multiple requests arrive during one active request, only the most
   // recent request will run (the others get overwritten).
   if (active_request_) {
-    pending_request_.reset(new SpellingRequest(text, std::move(callback)));
+    pending_request_ = std::move(incoming_request);
     return;
   }
 
-  active_request_.reset(new SpellingRequest(text, std::move(callback)));
+  active_request_ = std::move(incoming_request);
 
   JNIEnv* env = base::android::AttachCurrentThread();
   Java_SpellCheckerSessionBridge_requestTextCheck(