Fix build breakage at r30320.
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: http://src.chromium.org/svn/trunk/src@30321 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 8db0372a8f6054563ed232f3aedaab7311edd0c9
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser