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(
+        base::Time::FromString(last_modified.c_str(), &last_modified_time));

[digit1, 2017/01/03 15:29:13]

note: I'm not sure this is the right thing to do here. What could be the impact
of an invalid modified-time string here? It looks like the extracted base::Time
value is stored in a cache node. So the worst that could happen would be to
force too many re-downloads of a resource. However that's my very superficial
view of the code. Would love to have a better opinion on this, and be able to
add a comment here explaining why it's ok.

At least the change doesn't change runtime behaviour.

 
     bool has_no_store_header = false;
     std::string cache_control =