Independently enable recording and event callback

Previously we count TraceLog::SetEnabled() and SetDisabled() to allow
nested calls from about:tracing and Timeline panel of the DevTools.
Actually DevTools only needs the event callbacks, so we can allow the
two features (recording for about:tracing, event callback for DevTools)
to be independently enabled.

Also don't count TraceLog::SetEnabled() and SetDisabled() to ease
trace-startup and simplify the API semantics.

BUG=none
TEST=TraceEventCallbackTest.TraceEventCallbackAndRecording*, TraceEventTestFixture.*
R=caseq@chromium.org, nduca@chromium.org, pfeldman@chromium.org, tony@chromium.org
TBR=sky (for trivial call site change in content/browser/browser_shutdown_profile_dumper.cc)

Review URL: https://codereview.chromium.org/66193005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236078 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236078
Added hack to prevent maps scaling

BUG=143317
R=kbr@chromium.org

Review URL: https://codereview.chromium.org/75543012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236079 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236079