Add a use counter and histogram to FileReaderSync.

third_party/WebKit/Source/core/fileapi/FileReaderSync.cpp

Index: third_party/WebKit/Source/core/fileapi/FileReaderSync.cpp
diff --git a/third_party/WebKit/Source/core/fileapi/FileReaderSync.cpp b/third_party/WebKit/Source/core/fileapi/FileReaderSync.cpp
index 2b967df96e5a43a235f75466b039ad1598954ae1..85cba56d496fcc8ac24c092bd51518ffaf93817c 100644
--- a/third_party/WebKit/Source/core/fileapi/FileReaderSync.cpp
+++ b/third_party/WebKit/Source/core/fileapi/FileReaderSync.cpp
@@ -36,10 +36,36 @@
 #include "core/fileapi/Blob.h"
 #include "core/fileapi/FileError.h"
 #include "core/fileapi/FileReaderLoader.h"
+#include "platform/Histogram.h"
 
 namespace blink {
 
-FileReaderSync::FileReaderSync() {}
+namespace {
+// These values are written to logs.  New enum values can be added, but existing
+// enums must never be renumbered or deleted and reused.
+enum class WorkerType {
+  DEDICATED_WORKER = 0,
+  SHARED_WORKER = 1,
+  SERVICE_WORKER = 2,
+  OTHER = 3,

[kinuko, 2017/02/10 00:41:38]

nit: as far as we follow the above comment values are guaranteed to be 0, 1, 2,
..., do we still want to be explicit?

[Marijn Kruisselbrink, 2017/02/10 17:42:53]

The documentation at
https://chromium.googlesource.com/chromium/src.git/+/HEAD/tools/metrics/histo...
says yes, we do.

+  MAX
+};
+}  // namespace
+
+FileReaderSync::FileReaderSync(ExecutionContext* context) {
+  WorkerType type = WorkerType::OTHER;
+  if (context->isDedicatedWorkerGlobalScope())
+    type = WorkerType::DEDICATED_WORKER;
+  else if (context->isSharedWorkerGlobalScope())
+    type = WorkerType::SHARED_WORKER;
+  else if (context->isServiceWorkerGlobalScope())
+    type = WorkerType::SERVICE_WORKER;
+  DEFINE_THREAD_SAFE_STATIC_LOCAL(
+      EnumerationHistogram, workerTypeHistogram,
+      new EnumerationHistogram("FileReaderSync.WorkerType",
+                               static_cast<int>(WorkerType::MAX)));
+  workerTypeHistogram.count(static_cast<int>(type));
+}
 
 DOMArrayBuffer* FileReaderSync::readAsArrayBuffer(
     ScriptState* scriptState,