iframe omnibar: revert line-height of title line
iframe omnibar: revert line-height of title line

This partly reverts "noop: change for UI"
    (commit 44015f89b1683fa77274dc379f367ab88d53fa59),
because the margin of top and bottom lines looked too wide.

gdh1995/vimium-plus,gdh1995/vimium-plus,gdh1995/vimium-plus