Add histograms for ExternalDataUseObserver

chrome/android/java/src/org/chromium/chrome/browser/datausage/ExternalDataUseObserver.java

Index: chrome/android/java/src/org/chromium/chrome/browser/datausage/ExternalDataUseObserver.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/datausage/ExternalDataUseObserver.java b/chrome/android/java/src/org/chromium/chrome/browser/datausage/ExternalDataUseObserver.java
index 141623adf039107b46b18846423471283744329f..ffb3a39281fa41037a14966acc383c49e4bbbabb 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/datausage/ExternalDataUseObserver.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/datausage/ExternalDataUseObserver.java
@@ -88,8 +88,10 @@ public class ExternalDataUseObserver {
      * @param bytesDownloaded number of bytes downloaded by Chromium.
      * @param bytesUploaded number of bytes uploaded by Chromium.
      * The result of this request is returned asynchronously via
-     * {@link #nativeOnReportDataUseDone}. A new report should be submitted only after the
-     * result has been returned via {@link #nativeOnReportDataUseDone}.
+     * {@link #nativeOnReportDataUseDone}. A new report should preferably be submitted only after
+     * the result of the previous report has been returned via {@link #nativeOnReportDataUseDone}.
+     * Submitting another data use report while the previous is pending may cause the previous
+     * report to be lost.
      */
     @CalledByNative
     protected void reportDataUse(String label, int networkType, String mccMnc,