DevTools: Fix test that rely on network resources content caching.

BUG=307628
TBR=pfeldman
NOTRY=true

Review URL: https://codereview.chromium.org/350833003

git-svn-id: svn://svn.chromium.org/blink/trunk@177226 bbb929c8-8fbe-4397-9dbb-9b2b20218538