Use ThreadData::Now() for TrackingInfo::time_posted initialization

TrackingInfo::time_posted is only used for task profiling, however, it
is always initialized with TimeTicks::Now(). As TimeTicks::Now() can be
relatively expensive, particularly on Android, instead use
ThreadData::Now(), effectively a no-op when task profiling is disabled.

With task profiling disabled by default on Android, this saves ~1-2us
per task, or ~40% of the fixed cost of task posting.

BUG=417156,405348

Review URL: https://codereview.chromium.org/846663002

Cr-Commit-Position: refs/heads/master@{#310853}

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