Fix reflow for mobile sites.

1. use correct mDefaultScale instead of 1.0f.
2. ignore mMinZoomScaleFixed in case of using wide viewport since
sometimes mobile site will impose a min scale while the web content
will make it bigger than overview scale.

issue: 4257815

Change-Id: Ib4ca9b4dab8585827081a32432076eeeaedecd38