[Android WebView] Simplify processing of initial-scale=0 with viewportMetaZeroValuesQuirk

It turns out that we can always treat zero initial-scale as 'auto'.
The previous logic has been copied from the current implementation,
but as experiments show, the resulting layout parameters are also
compatible with the existing implementation even with this simplified
logic.

BUG=282130

Review URL: https://codereview.chromium.org/25749002

git-svn-id: svn://svn.chromium.org/blink/trunk@158946 bbb929c8-8fbe-4397-9dbb-9b2b20218538