DevTools: NetworkRequest.asDataUrl should always use encoded content.

Otherwise URL might contain unexpectedely escaped chars.

BUG=427478

Review URL: https://codereview.chromium.org/675733004

git-svn-id: svn://svn.chromium.org/blink/trunk@184530 bbb929c8-8fbe-4397-9dbb-9b2b20218538