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This ensures that no PageGroupLoadDeferrers will be on the stack when
processing ViewMsg_SwapOut, which would lead to an ASSERT.

BUG=312490
TEST=Start a cross-process navigation and show a loop of alerts before commit.
R=cdn@chromium.org, darin@chromium.org

Review URL: https://codereview.chromium.org/50223002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233371 0039d316-1c4b-4281-b951-d872f2087c98