Minor cleanup to WebViewImpl::legibleScale

Note that textAutosizingFontScaleFactor defaults to 1, and even if
textAutosizingEnabled is false, if someone's set the fSF to a non-zero
value, it still makes sense for it to modify the max zoom.

R=aelias@chromium.org
BUG=

Review URL: https://codereview.chromium.org/66643002

git-svn-id: svn://svn.chromium.org/blink/trunk@162058 bbb929c8-8fbe-4397-9dbb-9b2b20218538