Introduce ThreadedInputConnection behind a switch

content/public/common/content_switches.cc

Index: content/public/common/content_switches.cc
diff --git a/content/public/common/content_switches.cc b/content/public/common/content_switches.cc
index b124c14d7bef8fbd1ced45a41bb5c842ec2bdb23..5c7a410adcf21c361c263d2a2107e3237063bea1 100644
--- a/content/public/common/content_switches.cc
+++ b/content/public/common/content_switches.cc
@@ -932,6 +932,11 @@ const char kRemoteDebuggingSocketName[]     = "remote-debugging-socket-name";
 // Block ChildProcessMain thread of the renderer's ChildProcessService until a
 // Java debugger is attached.
 const char kRendererWaitForJavaDebugger[] = "renderer-wait-for-java-debugger";
+
+// Use IME's own thread instead of using main UI thread. It also means that
+// we will not use replica editor and do a round trip to renderer to synchronize
+// with Blink data.
+const char kUseImeThread[] = "use-ime-thread";
 #endif
 
 // Enable the aggressive flushing of DOM Storage to minimize data loss.