ChannelMojo: Grow IPC message upper limit to 128MB

There are assumptions that the IPC message limit is 128MB
so ChannelMojo has to allow messages in upto the message size.
Such large messages happen when IDB reads/writes huge records.

BUG=464151
R=agl@chromium.org

Review URL: https://codereview.chromium.org/1029703002

Cr-Commit-Position: refs/heads/master@{#322072}

content/app/mojo/mojo_init.cc

@@ -6,6 +6,7 @@
 
 #include "base/lazy_instance.h"
 #include "base/memory/scoped_ptr.h"
+#include "ipc/ipc_channel.h"
 #include "third_party/mojo/src/mojo/edk/embedder/configuration.h"
 #include "third_party/mojo/src/mojo/edk/embedder/embedder.h"
 #include "third_party/mojo/src/mojo/edk/embedder/simple_platform_support.h"
@@ -18,7 +19,7 @@ class MojoInitializer {
  public:
   MojoInitializer() {
     mojo::embedder::GetConfiguration()->max_message_num_bytes =
-        64 * 1024 * 1024;
+        IPC::Channel::kMaximumMessageSize;
     mojo::embedder::Init(scoped_ptr<mojo::embedder::PlatformSupport>(
         new mojo::embedder::SimplePlatformSupport()));
   }