Only enable the Dart timeline for benchmarks

- Print the event's map if we encounter an error in benchmark_app.
- Add --dart-timeline flag to Dart content handler.
- s/--complete-timeline/--dart-timeline.

Sometimes we were generating a gigabyte of JSON text resulting in a timeout. This should fix that.

R=zra@google.com

Review URL: https://codereview.chromium.org/1665823003 .
diff --git a/mojo/tools/data/benchmarks b/mojo/tools/data/benchmarks
index 21e8cb5..4c1d39b 100644
--- a/mojo/tools/data/benchmarks
+++ b/mojo/tools/data/benchmarks
@@ -17,7 +17,7 @@
       }
     ],
     'shell-args': [
-      '--args-for=mojo:dart_content_handler --complete-timeline',
+      '--args-for=mojo:dart_content_handler --dart-timeline',
     ]
   },
   {
@@ -31,7 +31,7 @@
       }
     ],
     'shell-args': [
-      '--args-for=mojo:dart_content_handler --complete-timeline',
+      '--args-for=mojo:dart_content_handler --dart-timeline',
     ]
   },
   {
@@ -49,7 +49,7 @@
       }
     ],
     'shell-args': [
-      '--args-for=mojo:dart_content_handler --disable-observatory --complete-timeline',
+      '--args-for=mojo:dart_content_handler --disable-observatory --dart-timeline',
     ]
   },
   {
@@ -67,7 +67,7 @@
       }
     ],
     'shell-args': [
-      '--args-for=mojo:dart_content_handler --run-on-message-loop --complete-timeline',
+      '--args-for=mojo:dart_content_handler --run-on-message-loop --dart-timeline',
     ]
   },
   {