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[tracing] Do not check enabled for filtering in TRACE_EVENT_CATEGORY_GROUP_ENABLED

The TRACE_EVENT_CATEGORY_GROUP_ENABLED sets enabled to true even when
filtering mode was turned on. This should be set only for recording and
etw modes.

BUG=669611, 670013

Review-Url: https://codereview.chromium.org/2547193002
Cr-Commit-Position: refs/heads/master@{#437686}