Add metrics for triggering of JS dialogs on Android.

chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java

Index: chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java b/chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java
index a4b3825d3c6a5c98095429f34d39c8a8fdf36c54..d96ad0d76f4d109138440e7490b9c0724408744d 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java
@@ -17,6 +17,7 @@ import android.net.Uri;
 import android.os.Bundle;
 import android.provider.Browser;
 import android.support.annotation.Nullable;
+import android.support.v4.view.ViewCompat;
 import android.text.TextUtils;
 import android.util.Log;
 import android.view.ContextThemeWrapper;
@@ -2238,6 +2239,16 @@ public class Tab implements ViewGroup.OnHierarchyChangeListener,
     }
 
     /**
+     * @return Whether the tab can currently be interacted with by the user.  This requires the
+     *         view to be visible and in a state the user can manipulate the tab (i.e. not in
+     *         something like the phone tab switcher).

[gone, 2017/01/13 18:54:37]

How does this work for a tablet, where the tab might be dead but is still
visible on the strip?  Maybe the comment can be clarified so that it implies
that the user is interacting with the web page displayed in the tab?

[Ted C, 2017/01/20 16:57:46]

Updated the comment to clarify the tab's view needs to be visible (since it
doesn't
necessarily need to be web contents).

+     */
+    @CalledByNative
+    public boolean isUserInteractable() {
+        return !mIsHidden && ViewCompat.isAttachedToWindow(getView());
+    }
+
+    /**
      * @return Whether or not the tab is in the closing process.
      */
     public boolean isClosing() {