Skip default categories when getting known categories.

We don't want to see the default categories when we get the known
categories as they aren't tracing categories that the user will
care about.

BUG=195661


Review URL: https://chromiumcodereview.appspot.com/12676007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@190675 0039d316-1c4b-4281-b951-d872f2087c98
2 files changed