Add ability to wire up sharelist in glcontext creation

include/gpu/gl/SkGLContext.h

Index: include/gpu/gl/SkGLContext.h
diff --git a/include/gpu/gl/SkGLContext.h b/include/gpu/gl/SkGLContext.h
index 77fd325dd853e9eac8d061316134474258135071..bdc1313d4632126555c5fc098a59550907cb044a 100644
--- a/include/gpu/gl/SkGLContext.h
+++ b/include/gpu/gl/SkGLContext.h
@@ -113,14 +113,17 @@ private:
     friend class GLFenceSync;  // For onPlatformGetProcAddress.
 };
 
-/** Creates platform-dependent GL context object
+/** Creates platform-dependent GL context object.  The shareContext parameter is in an optional
+ * context with which to share display lists. This should be a pointer to an SkGLContext created
+ * with SkCreatePlatformGLContext.  NULL indicates that no sharing is to take place.

[bsalomon, 2016/01/20 15:26:07]

Can you rewrap the rest of the comment to agree with your addition?

  * Returns a valid gl context object or NULL if such can not be created.
  * Note: If Skia embedder needs a custom GL context that sets up the GL
  * interface, this function should be implemented by the embedder.
  * Otherwise, the default implementation for the platform should be compiled in
  * the library.
  */
-SK_API SkGLContext* SkCreatePlatformGLContext(GrGLStandard forcedGpuAPI);
+SK_API SkGLContext* SkCreatePlatformGLContext(GrGLStandard forcedGpuAPI,
+                                              SkGLContext* shareContext = nullptr);
 
 /**
  * Helper macros for using the GL context through the GrGLInterface. Example: