Cleanup ipc_perftests.

-add an IO thread in child thread for the IPC case: this was missing and distorting results
-remove MojoChannelPerfTest.ChannelPingPong: this isn't representative of how IPC is used in chrome
-use idiomatic IPC macros: this doesn't change performance but is easier to read

Change-Id: I18fef0d766b2261ed8f6085fbc01d0215da6e1b4
Reviewed-on: https://chromium-review.googlesource.com/571327
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487277}

ipc/ipc_test.mojom

@@ -31,6 +31,8 @@ interface IndirectTestDriver {
 
 interface Reflector {
   Ping(string value) => (string value);
+  [Sync]
+  SyncPing(string value) => (string response);
   Quit();
 };