Prevent set mInitialScale being overridden by mDefaultScale.

If mInitialScale is set, use it as the default scale; otherwise,
it'll be overridden by default density in case of book app where
mobile fixed viewport is used; since today the default scale is used
to calculate the mobile fixed viewport.

issue: 4982074
Change-Id: I21c2bf41478bb97351cb3fb1cc3735ab7d75e164