Fix an assert failure in CXFA_DefFontMgr::GetDefaultFont()

Font information (like size) is needed to perform layout
in the process of loading XFA documents. It was too late
to set g_last_instance_id so that XFA didn't get a font
by calling FontMap().

BUG=pdfium:349

Review URL: https://codereview.chromium.org/1615723003

Cr-Commit-Position: refs/heads/master@{#372196}

pdf/pdfium/pdfium_engine.cc

@@ -606,6 +606,10 @@ PDFiumEngine::PDFiumEngine(PDFEngine::Client* client)
   IFSDK_PAUSE::version = 1;
   IFSDK_PAUSE::user = NULL;
   IFSDK_PAUSE::NeedToPauseNow = Pause_NeedToPauseNow;
+
+#if defined(OS_LINUX)
+  g_last_instance_id = client_->GetPluginInstance()->pp_instance();
+#endif
 }
 
 PDFiumEngine::~PDFiumEngine() {