Fix DCHECK failure when ChromeActivity#onPauseWithNative() is called many times quickly

chrome/browser/android/compositor/tab_content_manager.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/android/compositor/tab_content_manager.h"
 
 #include <android/bitmap.h>
 #include <stddef.h>
 
 #include <utility>
@@ skipping 199 matching lines @@
                                  const JavaParamRef<jobject>& tab,
                                  jfloat thumbnail_scale) {
   TabAndroid* tab_android = TabAndroid::GetNativeTab(env, tab);
   DCHECK(tab_android);
   int tab_id = tab_android->GetAndroidId();
   GURL url = tab_android->GetURL();
 
   content::WebContents* web_contents = tab_android->web_contents();
   DCHECK(web_contents);
 
+  if (pending_tab_readbacks_.find(tab_id) != pending_tab_readbacks_.end())
+    return;
+
   if (thumbnail_cache_->CheckAndUpdateThumbnailMetaData(tab_id, url)) {
     if (!web_contents->GetRenderViewHost() ||
         !web_contents->GetRenderViewHost()->GetWidget() ||
         !web_contents->GetRenderViewHost()
              ->GetWidget()
              ->CanCopyFromBackingStore() ||
-        pending_tab_readbacks_.find(tab_id) != pending_tab_readbacks_.end() ||
         pending_tab_readbacks_.size() >= kMaxReadbacks) {
       thumbnail_cache_->Remove(tab_id);
       return;
     }
 
     TabReadbackCallback readback_done_callback =
         base::Bind(&TabContentManager::PutThumbnailIntoCache,
                    weak_factory_.GetWeakPtr(), tab_id);
     pending_tab_readbacks_.set(
         tab_id, base::MakeUnique<TabReadbackRequest>(
@@ skipping 101 matching lines @@
            jint write_queue_max_size,
            jboolean use_approximation_thumbnail) {
   TabContentManager* manager = new TabContentManager(
       env, obj, default_cache_size, approximation_cache_size,
       compression_queue_max_size, write_queue_max_size,
       use_approximation_thumbnail);
   return reinterpret_cast<intptr_t>(manager);
 }
 
 }  // namespace android