PDF: Add UMA to track embedded PDF triggered drive-by downloads.

chrome/common/pdf_uma.cc

Index: chrome/common/pdf_uma.cc
diff --git a/chrome/common/pdf_uma.cc b/chrome/common/pdf_uma.cc
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+++ b/chrome/common/pdf_uma.cc
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+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "chrome/common/pdf_uma.h"
+
+#include "base/metrics/histogram_macros.h"
+
+// static
+void ReportPDFLoadStatus(PDFLoadStatus status) {
+  UMA_HISTOGRAM_ENUMERATION("PDF.LoadStatus", status, PDF_LOAD_STATUS_COUNT);
+}