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

chrome/common/pdf_uma.h

+// Copyright 2017 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.
+
+#ifndef CHROME_COMMON_PDF_UMA_H_
+#define CHROME_COMMON_PDF_UMA_H_
+
+// Must be kept in sync with PDFLoadStatus enum in histograms.xml.
+// This enum should be treated as append-only.
+enum class PDFLoadStatus {
+  kLoadedFullPagePdfWithPdfium = 0,
+  kLoadedEmbeddedPdfWithPdfium = 1,
+  kShowedDisabledPluginPlaceholderForEmbeddedPdf = 2,
+  kTriggeredNoGestureDriveByDownload = 3,
+  kPdfLoadStatusCount
+};
+
+void ReportPDFLoadStatus(PDFLoadStatus status);
+
+#endif  // CHROME_COMMON_PDF_UMA_H_