Refactor TabContentManager to async create ThumbnailCache.

chrome/browser/android/thumbnail/thumbnail_cache.cc

Index: chrome/browser/android/thumbnail/thumbnail_cache.cc
diff --git a/chrome/browser/android/thumbnail/thumbnail_cache.cc b/chrome/browser/android/thumbnail/thumbnail_cache.cc
index 130385de8c0f1c1fce774c6a1dded74f5fd86578..b9c403bd6755b5d2402e3227fbaa91459f5161cd 100644
--- a/chrome/browser/android/thumbnail/thumbnail_cache.cc
+++ b/chrome/browser/android/thumbnail/thumbnail_cache.cc
@@ -127,9 +127,7 @@ ThumbnailCache::ThumbnailCache(const std::string& disk_cache_path_str,
       cache_(default_cache_size),
       approximation_cache_(approximation_cache_size),
       ui_resource_provider_(NULL),
-      weak_factory_(this) {
-  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
-}
+      weak_factory_(this) {}
 
 ThumbnailCache::~ThumbnailCache() {
   SetUIResourceProvider(NULL);