CpuProfiler: replace FLAG_hydrogen_track_positions with is_tracking_positions method on CompilationInfo

this is the second part of https://codereview.chromium.org/1012633002.

almost mechanical change.
I'd like to enable positions tracking when cpu profiler is working.
But I'll switch it on for cpu-profiler in another patch.

BUG=chromium:452067
LOG=n

Review URL: https://codereview.chromium.org/995183005

Cr-Commit-Position: refs/heads/master@{#27224}