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As a side effect, this makes ctrl+click on a submit button open
a new background tab - similar to webkit and clicking on a link.

Also covered ctrl+click functionality with tests.

Bug: 849055
Change-Id: I344df4821a7c840129d239023d4a500a42e8f80b
Reviewed-on: https://chromium-review.googlesource.com/1100107Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567422}