Add API for flushing surfaces with gpu semaphores

BUG=skia:

Change-Id: Ia4bfef784cd5f2516ceccafce958be18a86f91d1
Reviewed-on: https://skia-review.googlesource.com/11488
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Forrest Reiling <freiling@google.com>
diff --git a/tests/OnFlushCallbackTest.cpp b/tests/OnFlushCallbackTest.cpp
index f4dd25f..81cd5da 100644
--- a/tests/OnFlushCallbackTest.cpp
+++ b/tests/OnFlushCallbackTest.cpp
@@ -9,6 +9,7 @@
 
 #if SK_SUPPORT_GPU
 
+#include "GrBackendSemaphore.h"
 #include "GrClip.h"
 #include "GrContextPriv.h"
 #include "GrDefaultGeoProcFactory.h"
@@ -575,7 +576,7 @@
         rtc->drawRect(GrNoClip(), std::move(paint), GrAA::kNo, SkMatrix::I(), r);
     }
 
-    rtc->prepareForExternalIO();
+    rtc->prepareForExternalIO(0, nullptr);
 
     SkBitmap readBack;
     readBack.allocN32Pixels(kFinalWidth, kFinalHeight);