Settings[DICE]: fix sign-in button spacing

Bug: 813716
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I44ca92b701d1fc1140bdbfb577fb9cb0a5680219
Reviewed-on: https://chromium-review.googlesource.com/953972


Commit-Queue: Scott Chen <scottchen@chromium.org>
Reviewed-by: Dave Schuyler <dschuyler@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541952}