Update source roller for electron31

Ignore gcs dependencies and log them to prepare-electron-source-tree.sh.

prepare-electron-source-tree.sh is placed in sources now because I think it's a little risky to generate it on the fly in prepare(). By placing it in the sources, packagers could have a chance to review its content to make sure it's safe and we can keep track of the ignored gcs dependencies in this way.

I updated a patch that no longer applies and added a new patch to make electron build.

Fix #1