[debugger] track debugger feature usage via histogram.

src/runtime/runtime-liveedit.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/runtime/runtime-utils.h"
 
 #include "src/arguments.h"
 #include "src/debug/debug.h"
 #include "src/debug/debug-frames.h"
 #include "src/debug/liveedit.h"
@@ skipping 211 matching lines @@
 // Compares 2 strings line-by-line, then token-wise and returns diff in form
 // of JSArray of triplets (pos1, pos1_end, pos2_end) describing list
 // of diff chunks.
 RUNTIME_FUNCTION(Runtime_LiveEditCompareStrings) {
   HandleScope scope(isolate);
   CHECK(isolate->debug()->live_edit_enabled());
   DCHECK(args.length() == 2);
   CONVERT_ARG_HANDLE_CHECKED(String, s1, 0);
   CONVERT_ARG_HANDLE_CHECKED(String, s2, 1);
 
+  isolate->debug()->feature_tracker()->Track(DebugFeatureTracker::kLiveEdit);
+
   return *LiveEdit::CompareStrings(s1, s2);
 }
 
 
 // Restarts a call frame and completely drops all frames above.
 // Returns true if successful. Otherwise returns undefined or an error message.
 RUNTIME_FUNCTION(Runtime_LiveEditRestartFrame) {
   HandleScope scope(isolate);
   CHECK(isolate->debug()->live_edit_enabled());
   DCHECK(args.length() == 2);
@@ skipping 17 matching lines @@
   // We don't really care what the inlined frame index is, since we are
   // throwing away the entire frame anyways.
   const char* error_message = LiveEdit::RestartFrame(it.frame());
   if (error_message) {
     return *(isolate->factory()->InternalizeUtf8String(error_message));
   }
   return heap->true_value();
 }
 }  // namespace internal
 }  // namespace v8