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KDE releases since 4.4 have had activated a "tab-grouping"
feature when users middle-click on window titlebars. Make
Chrome be slightly less jarring by doing nothing in response
to middle-clicks on KDE4 instead of lowering windows.

BUG=36586

Review URL: https://codereview.chromium.org/557503002

Cr-Commit-Position: refs/heads/master@{#293870}