DocsUI: Don't crash on invalid window sizes.

Our layout code asserts when passed an (invalid) width of 0.  The assert
is unnecessary as we already handle this situation by always laying out
the content pane with a column count of at least 1.

Also, per Steve, bump the minimum column count in grid mode to 2.

BUG=27150284

Change-Id: I8aa87beaaf4907ef5248999d0c590deef732589f