Add swapBuffer call to SkGLContextHelper.

include/gpu/gl/SkGLContextHelper.h

Index: include/gpu/gl/SkGLContextHelper.h
diff --git a/include/gpu/gl/SkGLContextHelper.h b/include/gpu/gl/SkGLContextHelper.h
index 97d416b34a7a5923c00ff1cbe9734787717134d0..8a4faa35cffd299cbc26bc19ecd0c8fe74c64594 100644
--- a/include/gpu/gl/SkGLContextHelper.h
+++ b/include/gpu/gl/SkGLContextHelper.h
@@ -34,6 +34,14 @@ public:
 
     virtual void makeCurrent() const = 0;
 
+    /**
+     * If the drawing surface provided by the platform is double buffered this
+     * call will cause the platform to swap with buffer is currently being
+     * targeted.  If the current surface does not include a back buffer, this
+     * call has no effect.

[bsalomon, 2013/10/09 18:17:08]

Maybe we should say that this is mostly used for scheduling work to the GPU
(since all of the subclasses create primary buffers that are tiny and not meant
to be rendered to).

+     */
+    virtual void swapBuffers() const = 0;
+
     bool hasExtension(const char* extensionName) const {
         SkASSERT(NULL != fGL);
         return fExtensions.has(extensionName);