Fix null pointer exception in NonOverlappingStack#getNonDyingTabCount()

Stack#mStackTabs is sometimes null, so we need to do a null pointer check in
NonOverlappingStack#getNonDyingTabCount() to avoid a crash. In particular, the
following sequence of steps currently causes a crash:

- Close all normal tabs and open an incognito tab
- Focus the incognito tab
- Tap the tab switcher button on the toolbar

This CL fixes this crash.

Bug: 831359,846266
Change-Id: I0a64e0134458af772f944d650daf57a0c0650d68
Reviewed-on: https://chromium-review.googlesource.com/1074219
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Commit-Queue: Ryan Landay <rlanday@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563254}

chrome/android/java/src/org/chromium/chrome/browser/compositor/layouts/phone/stack/NonOverlappingStack.java

@@ -67,6 +67,8 @@ public NonOverlappingStack(Context context, StackLayoutBase layout) {
     }
 
     private int getNonDyingTabCount() {
+        if (mStackTabs == null) return 0;
+
         int dyingCount = 0;
         for (int i = 0; i < mStackTabs.length; i++) {
             if (mStackTabs[i].isDying()) dyingCount++;