[Dashboard] Add test_path_list mode for chart-container.sendGraphJsonRequest
[Dashboard] Add test_path_list mode for chart-container.sendGraphJsonRequest

This is a prerequisite for chart-container to be able to make use of the test
path lists that /list_tests's test_path_dict mode returns. It allows us to
thread lists of test paths directly through to /graph_json, which can be used
once chart-container thinks in terms of these lists internally. More details
on what must be done in order for that to happen can be found in the bug.

BUG=catapult:#3303

Review-Url: https://codereview.chromium.org/2718963005

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