Fix build breakage at r30320.

Fix warning: comparison between signed and unsigned integer expressions
in base/pickle.cc

TBR=thesig
BUG=none
TEST=build sucess

Review URL: http://codereview.chromium.org/342014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@30321 0039d316-1c4b-4281-b951-d872f2087c98

base/pickle.cc

@@ -369,7 +369,7 @@ char* Pickle::BeginWriteData(int length) {
 }
 
 void Pickle::TrimWriteData(int new_length) {
-  DCHECK_NE(variable_buffer_offset_, 0);
+  DCHECK_NE(variable_buffer_offset_, 0U);
 
   // Fetch the the variable buffer size
   int* cur_length = reinterpret_cast<int*>(