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Webview attributes are now only stored in one location (in the webview node), so that all the code needed to sync up the copies could be removed (and was).

Also reworked the behavior of the |allowtransparency| and |autosize| to be both consistent with each other and more intuitive in general (they are both treated like booleans now). I updated the tests to reflect this new behavior.

Review URL: https://codereview.chromium.org/698973003

Cr-Commit-Position: refs/heads/master@{#302661}