AU: locate bsdiff/bspatch in $PATH, rather than hard coding their location.

BUG=8484
TEST=overloaded bs{diff,patch} with a shell script wrapper, put it
early in $PATH, ran unittests, and verified my wrapper was called.

Change-Id: I16c6a9cea6d7268f7a0ae423fc04c6025b18f0df

Review URL: http://codereview.chromium.org/4114006
diff --git a/delta_diff_generator.cc b/delta_diff_generator.cc
index ea65c1a..b2d4c69 100644
--- a/delta_diff_generator.cc
+++ b/delta_diff_generator.cc
@@ -1632,8 +1632,8 @@
   return true;
 }
 
-const char* const kBsdiffPath = "/usr/bin/bsdiff";
-const char* const kBspatchPath = "/usr/bin/bspatch";
+const char* const kBsdiffPath = "bsdiff";
+const char* const kBspatchPath = "bspatch";
 const char* const kDeltaMagic = "CrAU";
 
 };  // namespace chromeos_update_engine