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This CL applies the default system font style to extension pages.
Extensions that specify their own fonts will still override these styles. 

BUG=141703, 395319

Review URL: https://codereview.chromium.org/400343002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284597 0039d316-1c4b-4281-b951-d872f2087c98