Fix leak in MetricsLogTest.InitialLogStabilityMetrics test.

MetricsLog posts a task to the blocking pool, via
MetricsNetworkObserver::ProbeWifiPHYLayerProtocol() which
wasn't being cleaned up. Use TestBrowserThreadBundle
instead of the deprecated TestBrowserThread to ensure
all posted tasks are cleaned up.

Also, removes suppressions for this leak.

BUG=328987
TBR=isherman@chromium.org

Review URL: https://codereview.chromium.org/116063003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241429 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241429