Telemetry / Android: don't try to get stack trace when browser is None.
Telemetry / Android: don't try to get stack trace when browser is None.

Due to adb flakyness, sometimes browser isn't started: don't try to get crash stack
in that case.

BUG=266331

Review URL: https://chromiumcodereview.appspot.com/21058007

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@214795 0039d316-1c4b-4281-b951-d872f2087c98

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