Add WARN_UNUSED_RESULT to base::Time methods that return bool.

components/nacl/renderer/ppb_nacl_private_impl.cc

Index: components/nacl/renderer/ppb_nacl_private_impl.cc
diff --git a/components/nacl/renderer/ppb_nacl_private_impl.cc b/components/nacl/renderer/ppb_nacl_private_impl.cc
index d39b2b701e22b1567a8ed474a2c7ae06e1e05e7a..7f4785a7f3ac33f3a54467162d3ce28f01583d1e 100644
--- a/components/nacl/renderer/ppb_nacl_private_impl.cc
+++ b/components/nacl/renderer/ppb_nacl_private_impl.cc
@@ -1623,7 +1623,8 @@ class PexeDownloader : public blink::WebAssociatedURLLoaderClient {
     std::string last_modified =
         response.httpHeaderField("last-modified").utf8();
     base::Time last_modified_time;
-    base::Time::FromString(last_modified.c_str(), &last_modified_time);
+    ignore_result(

[miu, 2017/01/10 04:02:42]

Did we check what downstream code will do if |last_modified_time| is not
parseable? If so, can you add a quick comment here explaining what we're
ignoring the result because ...?

[digit1, 2017/01/10 13:55:58]

I've checked downstream that the only user for this value handles null
base::Time values properly. I've added a comment explaining this in my latest
upload. Thanks.

+        base::Time::FromString(last_modified.c_str(), &last_modified_time));
 
     bool has_no_store_header = false;
     std::string cache_control =