DevTools: fix FrameResource.lastModified to be optional when not present

third_party/WebKit/Source/core/inspector/InspectorPageAgent.cpp

Index: third_party/WebKit/Source/core/inspector/InspectorPageAgent.cpp
diff --git a/third_party/WebKit/Source/core/inspector/InspectorPageAgent.cpp b/third_party/WebKit/Source/core/inspector/InspectorPageAgent.cpp
index f0f7669c868d0aa7f6d2f852119c60b0291c3740..64ae29daaaf159d13f9b8b34bb5b18b9b961d7d1 100644
--- a/third_party/WebKit/Source/core/inspector/InspectorPageAgent.cpp
+++ b/third_party/WebKit/Source/core/inspector/InspectorPageAgent.cpp
@@ -781,9 +781,11 @@ InspectorPageAgent::buildObjectForFrameTree(LocalFrame* frame) {
             .setUrl(urlWithoutFragment(cachedResource->url()).getString())
             .setType(cachedResourceTypeJson(*cachedResource))
             .setMimeType(cachedResource->response().mimeType())
-            .setLastModified(cachedResource->response().lastModified())
             .setContentSize(cachedResource->response().decodedBodyLength())
             .build();
+    double lastModified = cachedResource->response().lastModified();
+    if (!std::isnan(lastModified))
+      resourceObject->setLastModified(lastModified);
     if (cachedResource->wasCanceled())
       resourceObject->setCanceled(true);
     else if (cachedResource->getStatus() == ResourceStatus::LoadError)