[DevTools] Access cpu profiler only when JS capability is present.

This patch:
- removes direct access to profilerAgent;
- only creates CPUProfilerModel when JS capability is present;
- migrates some target observers to CPUProfilerModel observers;
- sets sampling interval immediately before recording.

BUG=none

Review-Url: https://codereview.chromium.org/2721663005
Cr-Commit-Position: refs/heads/master@{#453804}
8 files changed