DevTools: remove "type" getters in Resource and NetworkRequest.

Motivation:
1) Getters are evil
2) "type" is too generic
3) "contentType" method returns exactly the same value.

BUG=

Review URL: https://codereview.chromium.org/667743002

git-svn-id: svn://svn.chromium.org/blink/trunk@184218 bbb929c8-8fbe-4397-9dbb-9b2b20218538