Enables specifying mojo url mapping on command line

mojo/shell/mojo_url_resolver.cc

Index: mojo/shell/mojo_url_resolver.cc
diff --git a/mojo/shell/mojo_url_resolver.cc b/mojo/shell/mojo_url_resolver.cc
index c399854905e348b84ff7d0a074e853d7e23fa551..c179d75e8bde1a897bd81bcdcc06cba89b7ecd74 100644
--- a/mojo/shell/mojo_url_resolver.cc
+++ b/mojo/shell/mojo_url_resolver.cc
@@ -70,14 +70,16 @@ void MojoURLResolver::AddLocalFileMapping(const GURL& mojo_url) {
 }
 
 GURL MojoURLResolver::Resolve(const GURL& mojo_url) const {
-  std::map<GURL, GURL>::const_iterator it = url_map_.find(mojo_url);
-  if (it != url_map_.end())
-    return it->second;
+  const GURL mapped_url(ApplyCustomMappings(mojo_url));
 
-  std::string lib = MakeSharedLibraryName(mojo_url.host());
+  // Continue resolving if the mapped url is a mojo: url.
+  if (mapped_url.scheme() != "mojo")
+    return mapped_url;
+
+  std::string lib = MakeSharedLibraryName(mapped_url.host());
 
   if (!base_url_.is_valid() ||
-      local_file_set_.find(mojo_url) != local_file_set_.end()) {
+      local_file_set_.find(mapped_url) != local_file_set_.end()) {
     // Resolve to a local file URL.
     return default_base_url_.Resolve(lib);
   }
@@ -86,5 +88,16 @@ GURL MojoURLResolver::Resolve(const GURL& mojo_url) const {
   return base_url_.Resolve(lib);
 }
 
+GURL MojoURLResolver::ApplyCustomMappings(const GURL& url) const {
+  GURL mapped_url(url);
+  for (;;) {
+    std::map<GURL, GURL>::const_iterator it = url_map_.find(mapped_url);
+    if (it == url_map_.end())
+      return mapped_url;
+    mapped_url = it->second;
+  }
+  return mapped_url;
+}
+
 }  // namespace shell
 }  // namespace mojo