[NTP::SectionOrder] Propagate category rank from UI through the bridge.

chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/SectionList.java

Index: chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/SectionList.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/SectionList.java b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/SectionList.java
index c4ecfd880faeb8c229a9edc9d5489d7c293eb21d..f75a9f3f8537b4920e58d24840cff1c6c54bf487 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/SectionList.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/SectionList.java
@@ -51,7 +51,7 @@ public class SectionList
         SuggestionsSource suggestionsSource = mNewTabPageManager.getSuggestionsSource();
         int[] categories = suggestionsSource.getCategories();
         int[] suggestionsPerCategory = new int[categories.length];
-        int i = 0;
+        int categoryIndex = 0;
         for (int category : categories) {
             int categoryStatus = suggestionsSource.getCategoryStatus(category);
             if (categoryStatus == CategoryStatus.LOADING_ERROR
@@ -59,8 +59,11 @@ public class SectionList
                     || categoryStatus == CategoryStatus.CATEGORY_EXPLICITLY_DISABLED)
                 continue;
 
-            suggestionsPerCategory[i++] =
+            suggestionsPerCategory[categoryIndex] =
                     resetSection(category, categoryStatus, alwaysAllowEmptySections);
+            SuggestionsSection section = mSections.get(category);
+            if (section != null) section.setCategoryIndex(categoryIndex);

[dgn, 2017/01/17 17:53:18]

here, we can decide not to show a section because it's empty. In this case, we
increment categoryIndex and for UMA purpose we are going to have a hole in the
reported values. 

shouldn't we use mSections.size() here? 

+            ++categoryIndex;
         }
 
         mNewTabPageManager.trackSnippetsPageImpression(categories, suggestionsPerCategory);