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: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@30321 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser