PDF: Fix a crash from r372196 with the print preview PDFEngine client.

pdf/pdfium/pdfium_engine.cc

Index: pdf/pdfium/pdfium_engine.cc
diff --git a/pdf/pdfium/pdfium_engine.cc b/pdf/pdfium/pdfium_engine.cc
index 6295ce4632950308589e4d247dbf2b10b7e30f01..6f8f4b3ce9ee43b78cfe5b55697018f220bb2945 100644
--- a/pdf/pdfium/pdfium_engine.cc
+++ b/pdf/pdfium/pdfium_engine.cc
@@ -608,7 +608,10 @@ PDFiumEngine::PDFiumEngine(PDFEngine::Client* client)
   IFSDK_PAUSE::NeedToPauseNow = Pause_NeedToPauseNow;
 
 #if defined(OS_LINUX)
-  g_last_instance_id = client_->GetPluginInstance()->pp_instance();
+  // PreviewModeClient does not know its pp::Instance.
+  pp::Instance* instance = client_->GetPluginInstance();
+  if (instance)
+    g_last_instance_id = instance->pp_instance();
 #endif
 }