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This ensures that both infobars and bubbles are tested by creating both
DownloadTest.TestMultipleDownloadsInfobar and DownloadTest.
TestMultipleDownloadsBubble. The correct UI (infobar or bubble) is
forced with a switch.

BUG=438758

Review URL: https://codereview.chromium.org/976823002

Cr-Commit-Position: refs/heads/master@{#318987}