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This enables figuring out which thread is blocking shutdown just by looking
at the UI thread stack.

BUG=403610
R=sky@chromium.org

Review URL: https://codereview.chromium.org/478563002

Cr-Commit-Position: refs/heads/master@{#290015}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290015 0039d316-1c4b-4281-b951-d872f2087c98