Add WebAPK installation metrics.

chrome/browser/android/webapk/webapk_metrics.cc

+// 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/browser/android/webapk/webapk_metrics.h"
+
+#include "base/metrics/histogram_macros.h"

[dominickn, 2016/09/05 07:34:31]

If you're only using SPARSE_SLOWLY, then I think you can get rid of this
include.

[Xi Han, 2016/09/07 20:58:52]

I change to use UMA_HISTOGRAM_ENUMERATION.

+#include "base/metrics/sparse_histogram.h"
+
+namespace webapk {
+
+const char kDismissEventHistogram[] = "WebApk.Install.DismissEvent";
+const char kInstallEventHistogram[] = "WebApk.Install.InstallEvent";
+const char kUserActionHistogram[] = "WebApk.Install.UserAction";
+
+void TrackDismissEvent(int event) {

[dominickn, 2016/09/05 07:34:31]

You could make these take the enum argument directly, and avoid the need for the
DCHECKs? Again, the app banner code isn't an exemplar for this. :)

[Xi Han, 2016/09/07 20:58:52]

Done.

+  DCHECK_LT(DISMISS_EVENT_MIN, event);
+  DCHECK_LT(event, DISMISS_EVENT_MAX);
+  UMA_HISTOGRAM_SPARSE_SLOWLY(kDismissEventHistogram, event);
+}
+
+void TrackInstallEvent(int event) {
+  DCHECK_LT(INSTALL_EVENT_MIN, event);
+  DCHECK_LT(event, INSTALL_EVENT_MAX);
+  UMA_HISTOGRAM_SPARSE_SLOWLY(kInstallEventHistogram, event);
+}
+
+void TrackUserAction(int event) {
+  DCHECK_LT(USER_ACTION_MIN, event);
+  DCHECK_LT(event, USER_ACTION_MAX);
+  UMA_HISTOGRAM_SPARSE_SLOWLY(kUserActionHistogram, event);
+}
+
+}  // namespace webapk