Add swapBuffer call to SkGLContextHelper.

src/gpu/gl/unix/SkNativeGLContext_unix.cpp

Index: src/gpu/gl/unix/SkNativeGLContext_unix.cpp
diff --git a/src/gpu/gl/unix/SkNativeGLContext_unix.cpp b/src/gpu/gl/unix/SkNativeGLContext_unix.cpp
index e6fae8ed7b2a7704494c7d5f14bb94a64c5fa80c..c4bd6f937d35142aeaa9e5c2d7f5c124c9deb9d3 100644
--- a/src/gpu/gl/unix/SkNativeGLContext_unix.cpp
+++ b/src/gpu/gl/unix/SkNativeGLContext_unix.cpp
@@ -285,3 +285,7 @@ void SkNativeGLContext::makeCurrent() const {
         SkDebugf("Could not set the context.\n");
     }
 }
+
+void SkNativeGLContext::swapBuffers() const {
+    glXSwapBuffers(fDisplay, fGlxPixmap);
+}