The empty string means to fall back to the pref for the Common script ("Zyyy").
For example, if chrome.fonts.serif.Cyrl is the empty string, it means to use
chrome.fonts.serif.Zyyy for Cyrillic script. Prefs that are the empty string are
normally not passed to WebKit since there are so many of them that doing so
would cause a perf regression (see r164050 and its revert r164294). Not passing
the pref is normally okay since WebKit does the desired fallback behavior
regardless of whether the empty string is passed or the pref is not passed at
all. But if the pref has changed from non-empty to the empty string, we must
let WebKit know.

BUG=157558


Review URL: https://chromiumcodereview.appspot.com/11336008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@165125 0039d316-1c4b-4281-b951-d872f2087c98