Fix ArithmeticException when recording tab launch type

This CL fixes the arithmetic exception when recording tab launch type.
TasksUma::recordTAbLaunchType should return early when total tab count
is equal to zero.

Bug: 906616
Change-Id: I03c9d069b1292d3adc73f14b1b99bc50c1709392
Reviewed-on: https://chromium-review.googlesource.com/c/1343534Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Commit-Queue: Mei Liang <meiliang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610193}

chrome/android/java/src/org/chromium/chrome/browser/tasks/TasksUma.java

@@ -60,6 +60,8 @@ public class TasksUma {
         int othersCreatedCount = 0;
         int totalTabCount = model.getCount();
 
+        if (totalTabCount == 0) return;
+
         for (int i = 0; i < totalTabCount; i++) {
             Integer tabLaunchType = model.getTabAt(i).getLaunchTypeAtInitialTabCreation();
             if (tabLaunchType == null) {