Safebrowsing: grab lookup_lock_ when clearing browse_gethash_cache_ during update.

BUG=391467

Review URL: https://codereview.chromium.org/449283002

Cr-Commit-Position: refs/heads/master@{#288294}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288294 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/safe_browsing/safe_browsing_database.cc

@@ -1013,9 +1013,12 @@ bool SafeBrowsingDatabaseNew::UpdateStarted(
     return false;
   }
 
-  // Cached fullhash results must be cleared on every database update (whether
-  // successful or not.)
-  browse_gethash_cache_.clear();
+  {
+    base::AutoLock locked(lookup_lock_);
+    // Cached fullhash results must be cleared on every database update (whether
+    // successful or not.)
+    browse_gethash_cache_.clear();
+  }
 
   UpdateChunkRangesForLists(browse_store_.get(),
                             safe_browsing_util::kMalwareList,