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 9dac7b19e4339dc9b9f592b83a1aa16d5a69da5b..7f233424c2afc89b6c132a8976506f041b36f0e8 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;
@@ -2137,6 +2138,16 @@ public class Tab implements ViewGroup.OnHierarchyChangeListener,
     }
 
     /**
+     * @return Whether the tab can currently be interacted with by the user.  This requires the
+     *         view owned by the Tab to be visible and in a state where the user can interact with
+     *         it (i.e. not in something like the phone tab switcher).
+     */
+    @CalledByNative
+    public boolean isUserInteractable() {
+        return !mIsHidden && ViewCompat.isAttachedToWindow(getView());
+    }
+
+    /**
      * @return Whether or not the tab is in the closing process.
      */
     public boolean isClosing() {