Add UMA for incognito tab count in Android.

Tab.Count.Incognito UMA was previously added in crrev.com/c/1647854,
but the Android implementation was missing.
It is added in this CL.

Bug: 966747, 1014018
Change-Id: I1f51550d7ad6844a0f53039a00625d33b53fcf69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1863799
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Auto-Submit: Ramin Halavati <rhalavati@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706793}

chrome/browser/ui/android/tab_model/tab_model_jni_bridge.cc

@@ -72,6 +72,9 @@ void TabModelJniBridge::TabAddedToModel(JNIEnv* env,
   TabAndroid* tab = TabAndroid::GetNativeTab(env, jtab);
   if (tab)
     tab->SetWindowSessionID(GetSessionId());
+
+  if (IsOffTheRecord())
+    UMA_HISTOGRAM_COUNTS_100("Tab.Count.Incognito", GetTabCount());
 }
 
 int TabModelJniBridge::GetTabCount() const {