Revert of Update systrace and profile_chrome's default agent behavior

systrace/profile_chrome/chrome_tracing_agent.py

Index: systrace/profile_chrome/chrome_tracing_agent.py
diff --git a/systrace/profile_chrome/chrome_tracing_agent.py b/systrace/profile_chrome/chrome_tracing_agent.py
index 1e8895bd2354fb714515181495fff0c86aeb7d1e..656a559dcc4670e86f4d6eb398dc18dc1aaa836d 100644
--- a/systrace/profile_chrome/chrome_tracing_agent.py
+++ b/systrace/profile_chrome/chrome_tracing_agent.py
@@ -14,7 +14,7 @@
 from systrace import tracing_agents
 
 
-DEFAULT_CHROME_CATEGORIES = '_DEFAULT_CHROME_CATEGORIES'
+_DEFAULT_CHROME_CATEGORIES = '_DEFAULT_CHROME_CATEGORIES'
 _HEAP_PROFILE_MMAP_PROPERTY = 'heapprof.mmap'
 
 
@@ -163,7 +163,8 @@
                          'Chrome\'s default categories. Chrome tracing can be '
                          'disabled with "--categories=\'\'". Use "list" to '
                          'see the available categories.',
-                         metavar='CHROME_CATEGORIES', dest='chrome_categories')
+                         metavar='CHROME_CATEGORIES', dest='chrome_categories',
+                         default=_DEFAULT_CHROME_CATEGORIES)
   chrome_opts.add_option('--trace-cc',
                          help='Deprecated, use --trace-frame-viewer.',
                          action='store_true')