DevTools: add extension API to contribute trace events to timeline

third_party/WebKit/LayoutTests/inspector/extensions/extensions-timeline-api-expected.txt

 Tests timeline support in WebInspector Extensions API
 
  Started extension.
 Running tests...
 RUNNING TEST: extension_testTimeline
+TraceProvider:
 {
     onRecordingStarted : {
         addListener : <function>
         removeListener : <function>
     }
     onRecordingStopped : {
         addListener : <function>
         removeListener : <function>
     }
 }
+Provider short display name: extension trace provider
+Provider long display name: long extension name
 traceProvider.onRecordingStarted fired.
+TracingSession:
+{
+    complete : <function>
+}
 traceProvider.onRecordingStopped fired.
+Timeline Flame Chart
+Group: long extension name
+Level 0
+Extension record X 1 (hsla(261, 82%, 70%, 0.7))
+Extension record X 2 (hsla(230, 88%, 70%, 0.7))
+Level 1
+Extension record I 1 (hsla(298, 100%, 70%, 0.7))
+Extension record B+E (hsla(292, 100%, 70%, 0.7))
 All tests done.