third_party/protobuf: update README.chromium.
third_party/protobuf: update README.chromium.

I forgot to update the README.chromium in r62331 to mention that we have local
changes to this code now.

BUG=none
TEST=none

git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@79196 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