Fix round-off error in Vector's collapsibleTabs.js

Fix round-off error in Vector's collapsibleTabs.js

Values returned by jQuery's .width() are rounded off, whereas offset values
computed with .offset() are not. As a result, if the window is sized just
right, collapsibleTabs.js will decide it has enough room to expand a tab when
it really doesn't. It will then detect the tab overlap caused by squeezing in
an additional tab and collapse it. It gets stuck in an infinite loop doing
that.

We could replace usage of $(el).width() with el.getBoundingClientRect().width
and thereby make the width calculations precise, but I noticed that jQuery is
nervous enough about the availability of getBoundingClientRect that it includes
a check to make sure it is not undefined. Rather than run the risk of a
ReferenceError, we can simply require an additional extra pixel of space before
we decide to expand anything.

Bug: 58682
Change-Id: Ib2096894619b8343735de482ee8bfa20a7cd0f48