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In one of the test cases, showModal() was called after the dialog had already
been removed from the DOM from tearDown().

BUG=642574
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2341493003
Cr-Commit-Position: refs/heads/master@{#418953}