More simplification in service worker metrics

- Use the helpers in histogram_functions.h instead of using custom
  versions.
- Use kMaxValue version of UMA_HISTOGRAM_ENUM in one more place
- Return const char* for enum stringification helpers to reduce number
  of temporary strings.
- Use StrCat() to build dynamic histogram names to reduce number of
  temporary strings.

  Bug: none

Change-Id: I720d11ad16c245f3ea59d55c2b75af8914eea7e8
Reviewed-on: https://chromium-review.googlesource.com/1174014
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582877}

content/public/browser/service_worker_context.h

@@ -39,7 +39,7 @@ enum class StartServiceWorkerForNavigationHintResult {
   // Something failed.
   FAILED = 5,
   // Add new result to record here.
-  NUM_TYPES
+  kMaxValue = FAILED,
 };
 
 // Represents the per-StoragePartition service worker data.