Do not use raw protocol types in inspector/timeline/timeline-network-resource.html

In preparation to switching to tracing backend the test is changed to use higher level abstractions.

BUG=398785
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/480013002

git-svn-id: svn://svn.chromium.org/blink/trunk@180451 bbb929c8-8fbe-4397-9dbb-9b2b20218538