Show media and VTT text tracks in the devtools.

This also introduces a Resource::Media type to the fetcher, and makes text
track fetches use the existing (but apparently unused) Resource::TextTrack
type.

This change does the initial work of wiring Media and TextTrack
resources into the devtools and leaves a lot of room for future
improvements, such as media previews in the Network tab.

BUG=329817

Review URL: https://codereview.chromium.org/114123005

git-svn-id: svn://svn.chromium.org/blink/trunk@171279 bbb929c8-8fbe-4397-9dbb-9b2b20218538