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 99548dfc83517e5275ad2c009c98963c8c823ada..b2b5fab04ea8f8657be9a4e01a23dd5d93fdf537 100644
--- a/content/public/common/content_switches.cc
+++ b/content/public/common/content_switches.cc
@@ -935,6 +935,11 @@ 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.