MD History: Hide horizontal scroll bar of synced tabs sign in promo

On MD History synced tab sign in promo, after resizing the browser window,
two horizontal scroll bars will be seen at the bottom which appears weird.

This CL fixed the bug by hiding the horizontal scroll bar of sign in promo
section.

BUG=622995
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2094123002
Cr-Commit-Position: refs/heads/master@{#402107}
1 file changed