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1) This is discouraged because it prevents callers from benefiting from automatic coersion from string/char* types.
2) A follow-up CL (http://codereview.chromium.org/8659047/) will make StringPiece a template, and thus awkward to forward declare. The very small number of places that were appropriately forward declaring it do not justify writing a 'string_piece_forward.h'.

BUG=87634
R=willchan@chromium.org


Review URL: http://codereview.chromium.org/8817019

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@113404 0039d316-1c4b-4281-b951-d872f2087c98