Allow linux trace events to not be time-shifted.

chrome/browser/resources/tracing/linux_perf_importer.js

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * @fileoverview Imports text files in the Linux event trace format into the
  * timeline model. This format is output both by sched_trace and by Linux's perf
  * tool.
  *
  * This importer assumes the events arrive as a string. The unit tests provide
@@ skipping 294 matching lines @@
         if (!this.isAdditionalImport_)
           return;
 
         // Remove the newly imported CPU slices from the model.
         this.abortImport();
         return false;
       }
 
       // Shift all the slice times based on the sync record.
       var sync = this.clockSyncRecords_[0];
+      // NB: parentTS of zero denotes no times-shift; this is
+      // used when user and kernel event clocks are identical.
+      if (sync.parentTS == 0 || sync.parentTS == sync.perfTS)
+        return true;
       var timeShift = sync.parentTS - sync.perfTS;
       for (var cpuNumber in this.cpuStates_) {
         var cpuState = this.cpuStates_[cpuNumber];
         var cpu = cpuState.cpu;
 
         for (var i = 0; i < cpu.slices.length; i++) {
           var slice = cpu.slices[i];
           slice.start = slice.start + timeShift;
           slice.duration = slice.duration;
         }
@@ skipping 304 matching lines @@
   };
 
   tracing.TimelineModel.registerImporter(LinuxPerfImporter);
 
   return {
     LinuxPerfImporter: LinuxPerfImporter,
     _LinuxPerfImporterTestExports: TestExports
   };
 
 });