Tracing: resume targets when manager is ready to record next tracing session.
Tracing: resume targets when manager is ready to record next tracing session.

Rationale: resuming targets while tracing is not complete may be a confusing experience - ongoing completion of tracing will generally affect how browser operates. Bonus: we can easily determine if it is okay to record profile it targets are not suspended.

BUG=

Review URL: https://codereview.chromium.org/1031513003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@192365 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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