Fix new -Wnull-conversion violations detected by a newer clang.

chrome/common/zip_reader.cc

Index: chrome/common/zip_reader.cc
diff --git a/chrome/common/zip_reader.cc b/chrome/common/zip_reader.cc
index 898654bcdc8e8c51922ecab1e1590840fb578fd3..a1b5b1c36953e44b5e6977500cfee80aa08c6e95 100644
--- a/chrome/common/zip_reader.cc
+++ b/chrome/common/zip_reader.cc
@@ -148,9 +148,9 @@ bool ZipReader::OpenCurrentEntryInZip() {
                                            NULL,  // szComment.
                                            0);  // commentBufferSize.
   if (result != UNZ_OK)
-    return NULL;
+    return false;
   if (raw_file_name_in_zip[0] == '\0')
-    return NULL;
+    return false;
   current_entry_info_.reset(
       new EntryInfo(raw_file_name_in_zip, raw_file_info));
   return true;