[Android WebView] Add a quirk for merging contents of multiple meta viewport tags

This is to support mobile frameworks that dynamically add several
meta viewport tags. Instead of discarding the previous tag when
processing the new one, we should merge their values.

BUG=306548

Review URL: https://codereview.chromium.org/27327002

git-svn-id: svn://svn.chromium.org/blink/trunk@159869 bbb929c8-8fbe-4397-9dbb-9b2b20218538