History: Use fixed positioning to get correct layout for iOS 6 & 7.

Use 'position: fixed' to place the scrolling container. This is a
workaround for a known bug in Chrome for iOS 6 (http://crbug.com/295516)
which prevents the flexbox-based layout from working properly on iOS.

BUG=285689
R=sergiu@chromium.org

Review URL: https://codereview.chromium.org/23494064

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224719 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224719