Bug 853933 - split the multi-locale JSON dictionary
tests on Unagi, v1-train, Settings startup time (average on 5 tries):

   4 locales, without patch: 991 ms (1043+983+968+1011+949)
   4 locales, with patch:    977 ms (1020+989+932+934+1009)

  61 locales, without patch: 1928 ms (1971+1927+1942+1894+1907)
  61 locales, with patch:    1521 ms (1514+1542+1486+1555+1506)