Making the Mojo Channel Messages legacy mode dynamic.

mojo/edk/system/channel.cc

Index: mojo/edk/system/channel.cc
diff --git a/mojo/edk/system/channel.cc b/mojo/edk/system/channel.cc
index 8a44d36024e2346b3eab90a48f1eb77a35adcd63..9fc8cfcc884b94b4590c88c31db7c9790ffe0f0d 100644
--- a/mojo/edk/system/channel.cc
+++ b/mojo/edk/system/channel.cc
@@ -14,6 +14,7 @@
 #include "base/macros.h"
 #include "base/memory/aligned_memory.h"
 #include "base/process/process_handle.h"
+#include "mojo/edk/embedder/embedder_internal.h"
 #include "mojo/edk/embedder/platform_handle.h"
 
 #if defined(OS_MACOSX) && !defined(OS_IOS)
@@ -45,6 +46,8 @@ static_assert(offsetof(Channel::Message::LegacyHeader, message_type) ==
               "message_type should be at the same offset in both Header "
               "structs.");
 
+bool g_use_legacy_protocol = false;
+
 }  // namespace
 
 const size_t kReadBufferSize = 4096;
@@ -53,13 +56,10 @@ const size_t kMaxChannelMessageSize = 256 * 1024 * 1024;
 const size_t kMaxAttachedHandles = 128;
 
 Channel::Message::Message(size_t payload_size, size_t max_handles)
-#if defined(MOJO_EDK_LEGACY_PROTOCOL)
-    : Message(payload_size, max_handles, MessageType::NORMAL_LEGACY) {
-}
-#else
-    : Message(payload_size, max_handles, MessageType::NORMAL) {
-}
-#endif
+    : Message(payload_size,
+              max_handles,
+              g_use_legacy_protocol ? MessageType::NORMAL_LEGACY
+                                    : MessageType::NORMAL) {}
 
 Channel::Message::Message(size_t payload_size,
                           size_t max_handles,
@@ -395,6 +395,13 @@ ScopedPlatformHandleVectorPtr Channel::Message::TakeHandlesForTransport() {
 #endif
 }
 
+// static
+void Channel::Message::SetUseLegacyTransportProtocol(bool use_legacy_protocol) {
+  // Make sure this is called before mojo::edk::Init() is called.
+  DCHECK(!internal::g_core);
+  g_use_legacy_protocol = use_legacy_protocol;
+}
+
 #if defined(OS_WIN)
 // static
 bool Channel::Message::RewriteHandles(base::ProcessHandle from_process,