Safebrowsing: Fix crash if DownloadItem is destroyed before history check.

chrome/browser/safe_browsing/download_protection_service.cc

Index: chrome/browser/safe_browsing/download_protection_service.cc
diff --git a/chrome/browser/safe_browsing/download_protection_service.cc b/chrome/browser/safe_browsing/download_protection_service.cc
index ba255be92d30fec094e9e8dcfe14bd764256927a..30f2523adcc4a3b1dee68117ff172c565c557bdc 100644
--- a/chrome/browser/safe_browsing/download_protection_service.cc
+++ b/chrome/browser/safe_browsing/download_protection_service.cc
@@ -644,6 +644,9 @@ class DownloadProtectionService::CheckClientDownloadRequest
 
   void GetTabRedirects() {
     DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+    if (!service_)
+      return;
+
     if (!tab_url_.is_valid()) {
       SendRequest();
       return;
@@ -669,6 +672,8 @@ class DownloadProtectionService::CheckClientDownloadRequest
                          const history::RedirectList* redirect_list) {
     DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
     DCHECK_EQ(url, tab_url_);
+    if (!service_)
+      return;
 
     if (!redirect_list->empty()) {
       tab_redirects_.insert(