SideBySide2: Fix B at 100% when A is hidden

Chrome 34.0.1847.131 on Mac didn't give B 100% after hiding A unless
we also let the column header take 100% width.  Previously I had
tested this on Chrome on Linux, but that may have been a different
version of Chrome.

Set both rows to use 100% width on B when A is hidden, making the
layout more explicit to the browser.

Change-Id: I5e334f072563754eecacfe2e514c7987a2dd97b3
SideBySide2: Fix line length columnMargin to appear at correct column

This element is inside of the scrolling area, to the right of the line
number gutter managed by CodeMirror.  Adding the width of the gutter
is incorrect, moving the margin marker that many columns right.

Change-Id: Ieaea5a48815a145ee748af9b97c6dc589c60f5dd
Merge "SideBySide2: Fix B at 100% when A is hidden"
Merge "SideBySide2: Fix line length columnMargin to appear at correct column"