Disable Idle tracing by default

Idle tracing can cause trace and log noise and GC allocations  for
other tracing users, so disable it be default. 
A new --enable-idle-tracing command line flag is added to
re-enable this tracing when needed. (And doubles as a blunt tool to
avoid findbugs warnings about unreachable code).

BUG=271050

Review URL: https://codereview.chromium.org/23296003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230668 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230668