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

chrome/browser/android/compositor/tab_content_manager.cc

Index: chrome/browser/android/compositor/tab_content_manager.cc
diff --git a/chrome/browser/android/compositor/tab_content_manager.cc b/chrome/browser/android/compositor/tab_content_manager.cc
index 12f582fee61c43047cd94e278dafa07285caa170..bdbdf1a306f2459fae294abbe31be80b7748de3f 100644
--- a/chrome/browser/android/compositor/tab_content_manager.cc
+++ b/chrome/browser/android/compositor/tab_content_manager.cc
@@ -217,13 +217,15 @@ void TabContentManager::CacheTab(JNIEnv* env,
   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;