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We can trigger this case by adding a webview that is not in DOM through
newwindow API.

Fix so that deferred attachment would not throw error, this is done by
returning true in attachWindow() method in this case.

BUG=None
Test=Load a <webview> in a chrome app. Make a newwindow request from
that webview. When the newwindow event fires, call e.window.attach
with a <webview> that is not on DOM, this should work as intended,
when the later <webview> is attached to DOM, it should load properly.

Review URL: https://codereview.chromium.org/497833003

Cr-Commit-Position: refs/heads/master@{#291594}