[blink] Record filereader metrics using //base/metrics helpers.

This also converts the various metric enums to be scoped enums, which
allows:
- clang to enforce kMaxValue correctness
- autodeduction of the max value by UMA_HISTOGRAM_ENUMERATION.

Bug: 742517, 1047547
Change-Id: I3d7c4476d0001fd9984527918f7c2494332653a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2506359
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Auto-Submit: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822089}

third_party/blink/renderer/core/fileapi/file_reader_loader.cc

@@ -47,7 +47,6 @@
 #include "third_party/blink/renderer/core/loader/threadable_loader_client.h"
 #include "third_party/blink/renderer/core/typed_arrays/dom_array_buffer.h"
 #include "third_party/blink/renderer/platform/blob/blob_url.h"
-#include "third_party/blink/renderer/platform/instrumentation/histogram.h"
 #include "third_party/blink/renderer/platform/loader/fetch/fetch_initiator_type_names.h"
 #include "third_party/blink/renderer/platform/loader/fetch/resource_error.h"
 #include "third_party/blink/renderer/platform/loader/fetch/resource_loader_options.h"
@@ -213,14 +212,12 @@ void FileReaderLoader::Cleanup() {
 }
 
 void FileReaderLoader::Failed(FileErrorCode error_code, FailureType type) {
-  DEFINE_THREAD_SAFE_STATIC_LOCAL(EnumerationHistogram, failure_histogram,
-                                  ("Storage.Blob.FileReaderLoader.FailureType",
-                                   static_cast<int>(FailureType::kCount)));
   // If an error was already reported, don't report this error again.
   if (error_code_ != FileErrorCode::kOK)
     return;
   error_code_ = error_code;
-  failure_histogram.Count(static_cast<int>(type));
+  base::UmaHistogramEnumeration("Storage.Blob.FileReaderLoader.FailureType",
+                                type);
   Cleanup();
   if (client_)
     client_->DidFail(error_code_);

third_party/blink/renderer/core/fileapi/file_reader_loader.h

@@ -125,7 +125,7 @@ class CORE_EXPORT FileReaderLoader : public mojom::blink::BlobReaderClient {
     // into this bucket. If there are a large number of errors reported here,
     // then there can be a new enumeration reported for mojo pipe errors.
     kMojoPipeUnexpectedReadError = 10,
-    kCount
+    kMaxValue = kMojoPipeUnexpectedReadError,
   };
 
   void Cleanup();

third_party/blink/renderer/core/fileapi/file_reader_sync.cc

@@ -32,6 +32,7 @@
 
 #include <memory>
 
+#include "base/metrics/histogram_functions.h"
 #include "third_party/blink/renderer/core/execution_context/execution_context.h"
 #include "third_party/blink/renderer/core/fileapi/blob.h"
 #include "third_party/blink/renderer/core/fileapi/file_error.h"
@@ -39,7 +40,6 @@
 #include "third_party/blink/renderer/core/typed_arrays/dom_array_buffer.h"
 #include "third_party/blink/renderer/platform/bindings/exception_state.h"
 #include "third_party/blink/renderer/platform/bindings/script_state.h"
-#include "third_party/blink/renderer/platform/instrumentation/histogram.h"
 
 namespace blink {
 
@@ -51,7 +51,7 @@ enum class WorkerType {
   DEDICATED_WORKER = 1,
   SHARED_WORKER = 2,
   SERVICE_WORKER = 3,
-  MAX
+  kMaxValue = SERVICE_WORKER,
 };
 }  // namespace
 
@@ -64,10 +64,7 @@ FileReaderSync::FileReaderSync(ExecutionContext* context)
     type = WorkerType::SHARED_WORKER;
   else if (context->IsServiceWorkerGlobalScope())
     type = WorkerType::SERVICE_WORKER;
-  DEFINE_THREAD_SAFE_STATIC_LOCAL(
-      EnumerationHistogram, worker_type_histogram,
-      ("FileReaderSync.WorkerType", static_cast<int>(WorkerType::MAX)));
-  worker_type_histogram.Count(static_cast<int>(type));
+  base::UmaHistogramEnumeration("FileReaderSync.WorkerType", type);
 }
 
 DOMArrayBuffer* FileReaderSync::readAsArrayBuffer(