Revert of Simplify mojo_shell since it's now only used for Mandoline.

mojo/shell/context.cc

Index: mojo/shell/context.cc
diff --git a/mojo/shell/context.cc b/mojo/shell/context.cc
index fc56476f819ea3392cb56a58ddb5b08aa9d4b71b..fbde07dd2ab0e6510a0905073a08a30e56ffae25 100644
--- a/mojo/shell/context.cc
+++ b/mojo/shell/context.cc
@@ -30,6 +30,7 @@
 #include "mojo/services/tracing/tracing.mojom.h"
 #include "mojo/shell/application_manager/application_loader.h"
 #include "mojo/shell/application_manager/application_manager.h"
+#include "mojo/shell/command_line_util.h"
 #include "mojo/shell/filename_util.h"
 #include "mojo/shell/in_process_native_runner.h"
 #include "mojo/shell/out_of_process_native_runner.h"
@@ -84,6 +85,22 @@ bool ConfigureURLMappings(const base::CommandLine& command_line,
     resolver->AddOriginMapping(GURL(origin_mapping.origin),
                                GURL(origin_mapping.base_url));
 
+  if (command_line.HasSwitch(switches::kURLMappings)) {
+    const std::string mappings =
+        command_line.GetSwitchValueASCII(switches::kURLMappings);
+
+    base::StringPairs pairs;
+    if (!base::SplitStringIntoKeyValuePairs(mappings, '=', ',', &pairs))
+      return false;
+    using StringPair = std::pair<std::string, std::string>;
+    for (const StringPair& pair : pairs) {
+      const GURL from(pair.first);
+      const GURL to = context->ResolveCommandLineURL(pair.second);
+      if (!from.is_valid() || !to.is_valid())
+        return false;
+      resolver->AddURLMapping(from, to);
+    }
+  }
   return true;
 }