use StringPiece for makefile deps

use StringPiece for makefile deps

Because of this, MakefileParser now returns pointers into the source
makefile string rather than allocating new strings.  Despite needing
to take the result and stuff it into a new string anyway to canonicalize
it, this takes another 50ms or so off the null Chrome build, likely
due to the vector used in MakefileParser changing to a type that doesn't
use any allocations.

(I also experimented with making the vector reserve an initial size but
didn't see any performance impact.)