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After we load a <webview>, setting its display to "none" removes the
plugin <object> from the render tree. If we set the display back to "",
we need to properly detect the case and allocate new instance id
and call attach to make things work.

The problem is briefly described here:
https://docs.google.com/a/chromium.org/document/d/1ITPOxS97DsG1HkbTAlpH0YcaTXKZNYUs84p1MppaHg0/view

BUG=399060
Test=Load a <webview> in a chrome app. See it render.
Set <webview>.style.display = 'none', see it go away.
Set <webview>.stlye.display = '', see it render properly.

Review URL: https://codereview.chromium.org/440463002

Cr-Commit-Position: refs/heads/master@{#291990}