Start timeline recording using TimelinePanel methods instead of agent methods
Start timeline recording using TimelinePanel methods instead of agent methods

We are going to switch to tracing backend for the timeline and there won't be TimelineAgent so the tests are changed to use higher level abstractions.

BUG=398785

Review URL: https://codereview.chromium.org/480003002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@180438 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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