Rework UMAHistogramHelper and StatisticsDeltaReader into [Chrome]HistogramTester.

content/public/test/browser_test_utils.h

Index: content/public/test/browser_test_utils.h
diff --git a/content/public/test/browser_test_utils.h b/content/public/test/browser_test_utils.h
index 4c645011aae4fa311105f99f8804ccb9a5c1a7c4..8880fdad649f9e71f3ed6ae3f9e2399978cf28f7 100644
--- a/content/public/test/browser_test_utils.h
+++ b/content/public/test/browser_test_utils.h
@@ -188,6 +188,10 @@ bool SetCookie(BrowserContext* browser_context,
                const GURL& url,
                const std::string& value);
 
+// Fetch the histograms data from other processes. This should be called after
+// the test code has been executed but before performing assertions.
+void FetchHistogramsFromChildProcesses();
+
 // Watches title changes on a WebContents, blocking until an expected title is
 // set.
 class TitleWatcher : public WebContentsObserver {