Revert "REGRESSION(r157081): Fix multipart handling of ResourceFetcher"

r171969 broke content_browsertests WebNavigationApiTest.Failures on
Linux Tests (dbg), Win 7 (dbg) integration bots.

TBR=morrita@chromium.org
BUG=309641

Review URL: https://codereview.chromium.org/244413003

git-svn-id: svn://svn.chromium.org/blink/trunk@172010 bbb929c8-8fbe-4397-9dbb-9b2b20218538