Add a mode flag to the tracing framework.

The mode is provided when SetEnabled is called. Currently there
is only one mode which is the trace until buffer is full mode.

We will be adding a continuous tracing mode which this will support.

BUG=156025


Review URL: https://chromiumcodereview.appspot.com/12302036

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@184258 0039d316-1c4b-4281-b951-d872f2087c98
20 files changed