Add new test: DownloadTest.TestMultipleDownloadsBubble
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
Committed: https://crrev.com/
ff5eb76821e76cb6c90b940249caf3d748e6fbc0
Cr-Commit-Position: refs/heads/master@{#318987}
Review URL: https://codereview.chromium.org/
976823002
Cr-Commit-Position: refs/heads/master@{#319164}