[Resource-Timing] Fix `initiatorType` values

There were multiple issues with current `initiatorType` implementation:
* `fetch()` resources had an empty value instead of "fetch"
* `<video>` resources had an empty value instead of "video"
* `<audio>` resources had an empty value instead of "audio"
* `<track>` resources had "texttrack" value instead of "track"
* EventSource resources had an empty value instead of "other"

On top of that, the test to make sure the above is all in check had
some values that were not aligned to the spec, and was disabled due to
the flakiness of audio and video loads.

This CL fixes the above values, fixes the test to be more spec
compliant and enables it.

Bug: 649571, 822757
Change-Id: I1dc6fd35e5e4bc570cbbb7d743dc02279bf42b8b
Reviewed-on: https://chromium-review.googlesource.com/1040925
Commit-Queue: Yoav Weiss <yoav@yoav.ws>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Reviewed-by: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562221}
8 files changed