Add WebAPK installation metrics.

chrome/browser/android/webapk/webapk_metrics.h

+// 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.
+
+#ifndef CHROME_BROWSER_ANDROID_WEBAPK_WEBAPK_METRICS_H_
+#define CHROME_BROWSER_ANDROID_WEBAPK_WEBAPK_METRICS_H_
+
+namespace webapk {
+
+// The states of WebAPK installation process. These states are tracked to
+// record dismiss events for WebAPK installation.
+enum InstallState {
+  WAIT_FOR_START,
+  INSTALLING,
+  INSTALLED,
+};
+
+// Keep these enums up to date with tools/metrics/histograms/histograms.xml.
+// Events for WebAPKs installation flow. The sum of |InstalllEvent| histogram
+// is the total number of times that a WebAPK infobar was triggered.
+enum InstallEvent {

[dominickn, 2016/09/19 01:38:39]

I think if you get rid of the INSTALL_EVENT prefixes here that'll make it easier
to work out what is going on.

[Xi Han, 2016/09/20 18:25:18]

Done.

+  // Banner is deleted due to the closing of Chrome or the associated tab.

[dominickn, 2016/09/19 01:38:39]

Nit: change comment to "The user did not interact with the banner". 

Banners can also be closed by the user navigating from the page.

[Xi Han, 2016/09/20 18:25:18]

Acknowledged.

+  INSTALL_EVENT_BANNER_IGNORED,
+  // The banner with the "Add-to-Homescreen" button is dismissed before the
+  // installation started. "Dismiss" means the user closes the banner by
+  // clicking the "X" button.
+  INSTALL_EVENT_ADD_TO_HOME_SCREEN_DISMISS,

[dominickn, 2016/09/19 01:38:39]

Call this BANNER_DISMISSED_BEFORE_INSTALLATION.

[Xi Han, 2016/09/20 18:25:19]

Done.

+  // The banner with the "Adding" button is dismissed during installation.
+  INSTALL_EVENT_ADDING_DISMISS,

[dominickn, 2016/09/19 01:38:39]

Call this DISMISSED_DURING_INSTALLATION

[Xi Han, 2016/09/20 18:25:19]

Done.

+  INSTALL_EVENT_COMPLETED,
+  INSTALL_EVENT_FAILED,
+  INSTALL_EVENT_MAX,
+};
+
+// The ways of how WebAPK instllation is triggered / started.

[dominickn, 2016/09/19 01:38:39]

Nit: "The ways in which WebAPK installation can be started."

[Xi Han, 2016/09/20 18:25:19]

Done.

+enum StartType {
+  TRIGGERED_FROM_BANNER,

[dominickn, 2016/09/19 01:38:39]

TRIGGERED and STARTED are pretty difficult to tell apart from the names. From
what I can tell, TRIGGERED means that the banner was triggered, while STARTED
means that the user clicked INSTALL on the banner? These events seem like they
begin in different histograms: TRIGGERED seems to record that a WebAPK banner
was shown, and thus shouldn't be in this metric?

[Xi Han, 2016/09/20 18:25:19]

Hmm, how about introduce a "InstallTriggeredType" if that makes the meaning of
each metrics more clear? We do care about how many times that banner was shown
and times of installation is actually started in both ways. It might be less
confusing to have both TRIGGERED_FROM_BANNER &
TRIGGERED_FROM_ADD_TO_HOME_SCREEN_MENU. The only concern is the 
TRIGGERED_FROM_ADD_TO_HOME_SCREEN_MENU is the same measurement as
STARTED_FROM_ADD_TO_HOME_SCREEN_MENU.

+  STARTED_FROM_BANNER,
+  STARTED_FROM_ADD_TO_HOME_SCREEN_MENU,
+  START_TYPE_MAX,
+};
+
+// User actions after a WebAPK is installed.
+enum UserAction {
+  USER_ACTION_INSTALLED_OPEN,
+  USER_ACTION_INSTALLED_OPEN_DISMISS,
+  USER_ACTION_MAX,
+};
+
+void TrackInstallEvent(InstallEvent event);
+void TrackStartType(StartType event);
+void TrackUserAction(UserAction event);
+
+};  // namespace webapk
+
+#endif  // CHROME_BROWSER_ANDROID_WEBAPK_WEBAPK_METRICS_H_