Add swapBuffer call to SkGLContextHelper.

src/gpu/gl/mac/SkNativeGLContext_mac.cpp

Index: src/gpu/gl/mac/SkNativeGLContext_mac.cpp
diff --git a/src/gpu/gl/mac/SkNativeGLContext_mac.cpp b/src/gpu/gl/mac/SkNativeGLContext_mac.cpp
index 2c43c3d2d4cc2400d8186d2d700543911f14dd9b..df316d7ed1f12e00963afacffab3d3e788b7be88 100644
--- a/src/gpu/gl/mac/SkNativeGLContext_mac.cpp
+++ b/src/gpu/gl/mac/SkNativeGLContext_mac.cpp
@@ -39,6 +39,7 @@ const GrGLInterface* SkNativeGLContext::createGLContext() {
 #if MAC_OS_X_VERSION_10_7
         kCGLPFAOpenGLProfile, (CGLPixelFormatAttribute) kCGLOGLPVersion_3_2_Core,
 #endif
+        kCGLPFADoubleBuffer,
         (CGLPixelFormatAttribute)0
     };
     CGLPixelFormatObj pixFormat;
@@ -74,3 +75,7 @@ const GrGLInterface* SkNativeGLContext::createGLContext() {
 void SkNativeGLContext::makeCurrent() const {
     CGLSetCurrentContext(fContext);
 }
+
+void SkNativeGLContext::swapBuffers() const {
+    CGLFlushDrawable(fContext);
+}