Add ServeLocalDirectory to the shell abstraction.

mojo/devtools/common/devtoolslib/http_server.py

Index: mojo/devtools/common/devtoolslib/http_server.py
diff --git a/mojo/devtools/common/devtoolslib/http_server.py b/mojo/devtools/common/devtoolslib/http_server.py
index 012a3f4f5943d5d711e166b2c806fd1ea143016e..edaaf061e23595275d54bb334bdf8598ca85a7fc 100644
--- a/mojo/devtools/common/devtoolslib/http_server.py
+++ b/mojo/devtools/common/devtoolslib/http_server.py
@@ -126,12 +126,16 @@ def _GetHandlerClassForPath(base_path):
   return RequestHandler
 
 
-def StartHttpServer(path):
-  """Starts an http server serving files from |path| on random
-  (system-allocated) port. Returns the server address.
+def StartHttpServer(local_dir_path, host_port=0):
+  """Starts an http server serving files from |path| on |host_port|. Pass 0
+  as |host_port| to use a system-allocated port.
+
+  Returns:
+    Tuple of the server address and the port on which it runs.
   """
-  assert path
-  httpd = _SilentTCPServer(('127.0.0.1', 0), _GetHandlerClassForPath(path))
+  assert local_dir_path
+  httpd = _SilentTCPServer(('127.0.0.1', host_port),
+                           _GetHandlerClassForPath(local_dir_path))
   atexit.register(httpd.shutdown)
 
   http_thread = threading.Thread(target=httpd.serve_forever)