don't try to convert externalized string in Node

This fixes devtools crash. Fix rogerwang/node-webkit#396
disable checkes to download image data from canvas

Fix rogerwang/node-webkit#397
Revert "disable checkes to download image data from canvas"

This reverts commit 48a5e17c0372dcd4e4cc5543e5fdba1dd3940a55.
skip canvas origin check only in Node frame
snapshot for node-webkit
Merge pull request #1 from rogerwang/node

Update webkit from base