Use css-device-adapt constraining for legacy viewport tags.

This CL is trying to take the part of Kenneth's CL [1] that is about using
the constraining procedure from the CSS Device Adaptation spec for legacy
meta tags, as well as for @viewport descriptors, to better match the
specification. Descriptors translated from meta tags are not cascaded with
@viewport rules but are only applied if @viewport from author style is not
present. See thread [2] on www-style.

ViewportArguments::resolve had different modes for resolving viewport
descriptors depending on the source. This CL translates legacy meta viewport
tags into descriptor values as specified by the css-device-adapt spec, but
they are not cascaded together as currently spec'ed. The prioritization is
still the same, overwriting descriptors from lower priority sources.

XHTML MP viewport is implemented through UA styles using @viewport.

[1] https://codereview.chromium.org/19555002/
[2] http://lists.w3.org/Archives/Public/www-style/2012May/0371.html

Review URL: https://chromiumcodereview.appspot.com/23742003

git-svn-id: svn://svn.chromium.org/blink/trunk@157293 bbb929c8-8fbe-4397-9dbb-9b2b20218538