Fix trace file format for NaN/infinite values

JSON doesn't support NaN/Infinity.  When we get a non-finite float value
we should just emit null to be consistent with JavaScript.

Also removes a orphaned TraceEvent function declaration.

BUG=336787

Review URL: https://codereview.chromium.org/145013003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246419 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246419