Move nullable_string16.h to the string subdirectory.

BUG=247723
TEST=no change
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Review URL: https://codereview.chromium.org/16415016

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@206066 0039d316-1c4b-4281-b951-d872f2087c98
37 files changed