Minor cleanup to WebViewImpl::legibleScale
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@162058 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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