Bug 829912 - Check for null IME hints. r=jchen

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extra : rebase_source : b6183ac3c3d469d2401ac17b059b9564ef07f542
Bug 785918 - Part 2: Add MOZ_HAVE_CXX11_CONSTEXPR check and constexpr version of ArrayLength(). r=waldo

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extra : rebase_source : e826347bfdedc949614c9cca294858b7150dfda1