Add fence support for TransferBuffers

src/gpu/gl/GrGLAssembleInterface.cpp

Index: src/gpu/gl/GrGLAssembleInterface.cpp
diff --git a/src/gpu/gl/GrGLAssembleInterface.cpp b/src/gpu/gl/GrGLAssembleInterface.cpp
index e804dd605a4029b003e40e33afb59c49b67ff5ed..b435655a5f8777e5f1c121211b16c0654d307032 100644
--- a/src/gpu/gl/GrGLAssembleInterface.cpp
+++ b/src/gpu/gl/GrGLAssembleInterface.cpp
@@ -522,10 +522,16 @@ const GrGLInterface* GrGLAssembleGLInterface(void* ctx, GrGLGetProc get) {
         GET_EGL_PROC_SUFFIX(DestroyImage, KHR);
     }
 
-    if (glVer >= GR_GL_VER(4,0) || extensions.has("GL_ARB_sample_shading")) {
+    if (glVer >= GR_GL_VER(4, 0) || extensions.has("GL_ARB_sample_shading")) {
         GET_PROC(MinSampleShading);
     }
 
+    if (glVer >= GR_GL_VER(3, 2) || extensions.has("GL_ARB_sync")) {
+        GET_PROC(FenceSync);
+        GET_PROC(ClientWaitSync);
+        GET_PROC(DeleteSync);
+    }
+
     interface->fStandard = kGL_GrGLStandard;
     interface->fExtensions.swap(&extensions);
 
@@ -919,6 +925,12 @@ const GrGLInterface* GrGLAssembleGLESInterface(void* ctx, GrGLGetProc get) {
         GET_PROC_SUFFIX(MinSampleShading, OES);
     }
 
+    if (version >= GR_GL_VER(3, 0)) {
+        GET_PROC(FenceSync);
+        GET_PROC(ClientWaitSync);
+        GET_PROC(DeleteSync);
+    }
+
     interface->fStandard = kGLES_GrGLStandard;
     interface->fExtensions.swap(&extensions);