Add HTTP handler that redirects requests for Mojo apps

This CL adds an HTTP handler whose purpose is to redirect requests for Mojo
apps to the locations of the current versions of the binaries. It installs
itself as a handler for all requests and operates as follows:

(1) Ensure that the request is of the form "/platform/app", e.g.,
    "/linux-x64/view_manager.mojo".

(2) Load the file containing the up-to-date version of the requested app, which
    resides in a known location for the app and platform, e.g.,
"https://storage.googleapis.com/mojo/services/linux-x64/view_manager_location".
(3) Return a redirect to the contents of that file.

The expected usage of this app is to make it accessible from a server such as
https://foo.com and then to direct the Mojo shell to send requests for mojo:
apps to this server, e.g. "--origin=https://foo.com/linux-64". Note that the
platform must be specified explicitly since the shell does not send that
information at this time.

R=qsr@chromium.org

Review URL: https://codereview.chromium.org/953353002
diff --git a/services/BUILD.gn b/services/BUILD.gn
index 7abd919..355778a 100644
--- a/services/BUILD.gn
+++ b/services/BUILD.gn
@@ -31,6 +31,6 @@
   }
 
   if (is_linux) {
-    deps += [ "//services/python/content_handler" ]
+    deps += [ "//services/python" ]
   }
 }