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If we create a WebLocalFrame and then set blink's settings, the frame
gets an inconsistent view of the settings when they change out from
underneath it.

Concretely, this leads to us doing WebViewImpl::setRootGraphicsLayer(A)
with pinch virtual viewport disabled (the default in WebSettings), when
we attach the document to the tree, then we apply the prefs to enable
pinch virtual viewport, and later setRootGraphicsLayer(NULL) to detach
the document. This causes a crash because the viewport code expects
a matching attach before the detach.

With this CL, https://codereview.chromium.org/261143003 is able to pass
on the print preview and prerender browser tests.

R=bauerb@chromium.org
BUG=376531,361729

Review URL: https://codereview.chromium.org/296983017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@272628 0039d316-1c4b-4281-b951-d872f2087c98