Optimize memory usage in MultipartResponseDelegate by not using std::string::substr so much.
Optimize memory usage in MultipartResponseDelegate by not using
std::string::substr so much.

TEST=Covered by existing tests.

Review URL: http://codereview.chromium.org/322007

git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@29825 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